Multifunctional bag applicable to devices for transporting children Description Technical Field The present patent application generally relates to the field of accessories/fittings usable with products for childcare. The invention is applicable to any field where this kind of device can be advantageously used, but preferably it relates to the field of accessories for strollers or baby carriages. Background Art As known, strollers and baby carriages are very useful objects designed to render walks more pleasant and more comfortable, and to make life easier for parents. However, it frequently happens to take along small children or babies even during shopping. To ease this condition/situation, often on common shopping trolleys of the kind used in supermarkets, there are provided babies’ chairs on which small children may be put, so that they can be transported/displaced together with the trolley. However, taking for granted that this possibility is totally precluded to infants and newborns, who obviously necessitate of a transport modality more adequate to their particular needs in a baby carriage, also in the case of children parents are often doubtful concerning the use of baby chairs that are integrated in the aforementioned trolleys. In particular, this skepticism is a real consequence of the risks associated with the children’s health and the transport itself, due to problems of safety and concerns related to the hygiene of these accommodations. Moreover, it often happens that a sleeping child needs to be woken up in order to displace it, or in any case remove it, from the baby chair. As a consequence of what has been said it would be highly preferable during shopping to take along one’s child directly on one’s own stroller or baby carriage. Moreover, it is known to be very difficult for a person to push at the same time a shopping trolley and a stroller or baby carriage. While some of these baby carriages or strollers are already equipped with a storage space normally located below this kind of transport devices, such space seems insufficient for accommodating even a modest amount of purchased items. In the background art there exist storage nets designed to be fixed to the handles of said devices for babies’ transport, but they also have proven to be impractical and not capacious for the abovementioned purpose. Attempts were made to realize devices capable of solving the above problems and generally these consist of strollers and baby carriages originally designed to include storage spaces for shopping purposes having considerable dimensions or formed by real extensions, which are often equipped with their own wheels, so that they can be connected to a usual stroller o baby carriage. The latter devices, which form real shopping trolleys connectable to a baby carriage or stroller, allow assembling one device to the other in a single unit. GB 2403458, WO 97/29938 and WO 00/46091 are examples of such devices, wherein a trolley provided with a basket is adapted to be connected to a baby carriage or a stroller, at or adjacent the handles of the baby carriage or stroller. However, a drawback of these known devices is that shopping becomes complicated, since a baby carriage or stroller in front of a shopping trolley will assume dimensions such as to become too cumbersome to be pushed around in supermarket sections or, even worse, inside a shop. Instead, patent document GR 20190100216 relates to a stroller capable of transporting at the same time a newborn and shopping items; however, said stroller happens to be designed originally for this purpose, and consequently its disadvantage is that one is forced to use it even in situations where its cumbersome specific configuration would not be needed. Therefore, an object of the present invention generally consists in the realization of a bag usable in combination with any kind of common baby carriage or stroller for children, which is specifically designed to solve the above problems, as illustrated by the following detailed description of an illustrative and non-limitative embodiment thereof. Summary of the Invention The present patent application is intended to describe and claim a product endowed with at least one novel solution representing an alternative to the solutions known hitherto; in particular, the aim is to obviate to one or more drawbacks or problems discussed above, and/or to meet one or more needs already felt in this technical field, and in particular those derivable from the above ones. The main object of the present invention will therefore be to provide a bag that may be combined with any kind of common baby carriage or stroller, and at the same time to provide a sufficient storage space for the purchased products; thus, to provide a bag with as many functions as possible so as to be applicable/mountable on any kind of device for transporting children and, at the same time, which can be easily removed and carried using a comfortable shoulder strap. Moreover, another fundamental feature of the bag is the flexible structure of the materials of which it is formed, allowing the same to be folded in many parts and therefore easily stored. The present invention attains its main objects by realizing a bag having a prismatic shape, preferably with a rectangular base, and which comprises an upper face, a lower face, and four lateral faces. Said upper face is formed by an opening realized for stiffening the structure by a common memory wire, that is, capable of always recovering its original shape, so that said opening will be constantly maintained in a state ready to receive products which are introduced therein during shopping. Also said lower face is formed - in order to stiffen the structure - by a common memory wire, to which there is connected a common flexible textile structure of non-woven fabric suited to support the products’ weight, by sewing/stitching or any other fixing means. With regard to said four lateral faces, these are such as to allow observing the products stored inside; therefore, they may be transparent or multi-perforated, and are realized with a resistant layer of non-rigid plastic polymer. For what concerns the connection means used for the hooking/connection to the respective stroller or baby carriage, they have been designed specifically to be able to easily mount the innovative bag on any kind of existing stroller or baby carriage, that is, on one having any size whatever and with a single-piece handlebar or with separate handles. In particular, longitudinally of the lateral faces there are belts that may be hooked at their free ends using a common snap buckle having adjustment slits for the belt length, and wherein, upon hooking the belt it will rest on the entire length of the handle(bar) or will extend to the inside of the separate handles, if any. Another technical feature distinguishing the bag of the present invention is that said belts implement its multifunctional character, since – after their removal from the stroller or baby carriage - they may be adjusted in length by acting on said adjustment slits of the snap buckle and in this way they are used to form a comfortable shoulder strap which allows to easily carry the innovative bag together with the products present inside it. In order to improve/increase the stability and multifunctionality of the innovative bag when (for example) the innovative bag is mounted on baby carriages and strollers having a single- piece handle, said front and rear faces will be equipped with additional common belts whose free ends will present a Velcro portion to be fixed on the handlebar of the stroller or baby carriage, in such a way as to be adequately locked on the handlebar by means of a common tear-off closure system. An additional object is a further implementation of the multifunctional character of the bag by said lateral belts since when the bag is removed from the device for transporting children it can be easily folded (due to the specific flexible materials of its components) and be maintained in this more compact configuration/state by locking it with the aforesaid tear-off closure system of the belts with Velcro. Still a further object is to provide a multifunctional bag which may be produced on an industrial scale, whose components are standard and readily/directly available on the market, and whose production is completely sustainable from an economical and environmental viewpoint. Another feature of the multifunctional bag according to the present invention is that its fixing/locking and removal occurs in a simplest and safest way and will meet all safety regulations in the field, by acknowledging/incorporating predominantly the numerous existing rules concerning baby carriages and strollers. With regard to the dimensions of said inventive multifunctional bag these will be purely indicative and non-limiting for the invention; nonetheless, the overall dimensions of the upper and lower faces can be chosen to lie in an interval from 85 cm to 15 cm, preferably 50 cm, for the width, in an interval from 70 cm to 15 cm, preferably 30 cm, for the depth, and finally the height of the lateral sides will lie in an interval between 90 cm and 30 cm, preferably 60 cm. Other features of the present invention will be described in the following detailed description of one or more specific embodiments, which are protected by the various dependent claims. Brief Description of Drawings The abovementioned advantages as well as other features and advantages of the present invention will be disclosed with reference to the annexed drawings to be considered purely illustrative and non-limitative or binding for the present patent application, wherein: FIGURE 1 is a perspective view of the multifunctional bag according to the present invention, installed on a stroller; FIGURE 2 is a front view of the multifunctional bag, installed on a stroller; FIGURE 3 is a perspective view of the multifunctional bag; FIGURE 4 is a perspective view of the multifunctional bag according to an alternative embodiment of the present invention; FIGURE 5 is a perspective view of a common snap buckle with adjustment slits according to the present invention. Detailed Description of the Invention The present invention will now be described in detail with reference to the drawings and to a preferred embodiment thereof shown in them, wherein for the same components the same reference numerals have been used. However, it is emphasized that in the following only some of the possible embodiments of the invention will be disclosed for illustrative and non- limitative purposes, though many other ones could be described relying on the particular technical solutions envisaged. From Fig. 3 it can be seen that the multifunctional bag 100 has a prismatic shape with rectangular basis and is constituted by an upper face 110, a lower face 120 and four faces including two lateral faces 130, a front face 140 and a rear face 150. Said upper face 110 is formed by an aperture whose rectangular shape is induced by the presence of a common memory wire 111 capable of always recovering its original shape, so as to constantly preserve an open state of said opening and therefore capable of receiving the products to be introduced inside the bag during shopping. Figs. 2 and 3 make clear that also said lower face 120 has a rectangular shape induced by the presence of a common memory wire 121, but with the latter a base 122 is joined by sewing/stitching or any other kind of joining means, in such a way to be able to support the products’ weight. The aforesaid configuration of the multifunctional bag 100 is a first aspect of the present invention, while a second aspect relates instead to the ensemble formed by the connection means for the different portions of a device for transporting children, like a stroller or a baby carriage. It can be seen from the various figures that said connection means are constituted by lateral belts 330 longitudinally joined to said lateral faces 130, in particular inferiorly attached at one end to said memory wire 121 and upperly to the memory wire 111, while terminating at the other free end inside respective adjustment slits 210 used for adjusting the belt length and forming part of a common snap buckle 200, as shown in Fig. 5; wherein, said lateral belt 330, upon its connection/hooking, is adapted to be positioned across the whole length of a single handlebar, or (alternatively) within separate handles, if any. As shown in Fig. 5 it is preferable to use a usual snap buckle 200 to reversibly connect these two lateral belts 330 to each other, though this is not considered an essential feature for the protection of the present invention. Any other common belt length regulation and connecting/hooking system could be used alternatively. A further technical feature that distinguishes the bag of the present invention is the fact that the lateral belts 330 implement its multifunctional character, since after their removal from the stroller or baby carriage they can be adjusted in length by acting on said adjustment slits 210 of the snap buckle 200 (Fig.5) and be employed so as to form a convenient/comfortable shoulder strap, for an easy transportation of the innovative multifunctional bag 100 even when products are contained therein. With reference to the various figures, these show the multifunctional bag 100 in which additional connection means are provided in order to increase/improve its stability and multifunctional character, for instance in case of its application to/on strollers and baby carriages equipped with a single handlebar. Specifically, the front face 140 is longitudinally provided with a belt 340 while the rear face 150 longitudinally presents a belt 350, wherein both of said belts 340 and 350 are connected on their lower part – by means of one end thereof – to said memory wire 121 and on the upper part to the memory wire 111 and they respectively terminate (at the height of the corresponding free end) in a Velcro portion 370 (Fig.3) adapted to form a common tear-off closure system capable of attaining a locking on a suitable portion of the device for transporting children, such as the single large handle/handlebar shown in detail in Figs. 1 and 2. Therefore, a specific object of the present invention is to provide a multifunctional bag 100 which on the one hand is a very useful system mountable/applicable on any kind of device for transporting children and employable to introduce products purchased during shopping, and on the other, is a valid means used to transport said products after separating/disconnecting the bag from said device for transporting children, and which - last but not least – can be easily folded, due to its flexible components/materials, in order to store it in a condition of minimum size thanks to the locking attained with said common tear-off closure system of said Velcro 370 (Figs. 2 and 3) present on the belts 340 and 350. In general, any material forming the multifunctional bag 100 can be chosen by the skilled person in a manner to meet the technical requirements needed in this particular field, among which one can mention the weight resistance and the resistance to tearing forces exerted on these materials, flexibility, resistance to repeated washing, and a low weight. However, by way of a non-limitative example, said materials can be formed by synthetic fibers, derived from nylon, polyester (PET), and polypropylene. On the other hand, for what concerns the material forming the memory wire 121 and the memory wire 111, this can be selected among the usual memory materials, that is, those capable of always recovering their original shape, by way of non-limiting example the nickel-titanium alloy known under the name Nitinol. The present invention includes further alternative embodiments such as variants of the structure and shape of its components. For instance, even if so far a multifunctional bag 100 has been described which is provided with a single aperture/opening, as shown in Fig. 4, on the front face 140 it will be possible to realize at least one pocket 400 suited to form a useful structural element for receiving particular objects, which need a particular caution and protection (housing), such as smartphones, keys, cards, wallets, etc. An additional aspect on the sidelines of the technical solutions described so far but having a particular relevance for the practical use of the invention, is that the multifunctional bag 100 can be a useful tool even if the transport device is a wheelchair for disabled and elderly people or in any case a device for displacing people with reduced mobility. Obviously, the information given herein is purely illustrative and non-limiting for the scope of the present invention, since they serve the only purpose of allowing a skilled person to understand some possible applications and realizations of the invention. Therefore, various obvious modifications to the above illustrative embodiments - described with reference to the figures - could clearly by made by a person of average skill in the art, without departing from the present inventive concept on which the invention is based, as defined in the following dependent claims.