A structure for child safety seats comprising a seat rest, a backrest, two sides and a headrest, characterised in that the structure for child safety seat comprises: - a rigid frame comprising: a base frame, with sides (5 and 6) that constitute the respective guides for the movement of an Isofix mechanism having respective anchoring ends that fastens the seat to the car, a backwards frame tilted joined at the bottom to the front part of the base frame, and a vertical support solidly connected to the base frame and solidly connected to the tilted frame, wherein the vertical support is formed by two columns (7 and 8) parallel between them, presenting corresponding guides (9 and 10) for the safety straps for strapping the child into the seat and which are joined by means of an upper cross member having means for fastening the upper fixation strap of the seat to the car; - a vertical support and a pneumatic cylinder, wherein the tilted frame is related to the rear section of the base frame by its upper frame by means of the vertical support, and wherein the pneumatic cylinder is disposed inside the vertical support and projecting from the vertical support at the top to the headrest support; - a frame by which the seat rest is supported, wherein the frame is mounted on a mechanism which determines the vertical movement of the seat rest by actuating on a side controller - components that constitute the support of the sides and of the headrest consisting of angular plates installed in a vertically sliding arrangement in the rigid frame, wherein the angular plate corresponding to the headrest presents a U-shaped constitution and the angular plates corresponding to the sides are formed from corresponding angular pieces (14 and 15), which are installed with the possibility of side movement, such that the headrest and the sides may be raised as the child grows and, further, the sides are also separated in a lateral movement in accordance with the child's growth; and - whose rigid frame receives the coupling of components that constitute the exterior support of the constituent parts of the seat, including said components a base, a sides (18, 19) and a headrest, whose parts are formed from blocks of pressure-absorbing lightweight material. - a rigid frame comprising: a base frame, with sides (5 and 6) that constitute the respective guides for the movement of an Isofix mechanism having respective anchoring ends that fastens the seat to the car, a backwards frame tilted joined at the bottom to the front part of the base frame, and a vertical support solidly connected to the base frame and solidly connected to the tilted frame, wherein the vertical support is formed by two columns (7 and 8) parallel between them, presenting corresponding guides (9 and 10) for the safety straps for strapping the child into the seat and which are joined by means of an upper cross member having means for fastening the upper fixation strap of the seat to the car; a base frame, with sides (5 and 6) that constitute the respective guides for the movement of an Isofix mechanism having respective anchoring ends that fastens the seat to the car, a backwards frame tilted joined at the bottom to the front part of the base frame, and a vertical support solidly connected to the base frame and solidly connected to the tilted frame, wherein the vertical support is formed by two columns (7 and 8) parallel between them, presenting corresponding guides (9 and 10) for the safety straps for strapping the child into the seat and which are joined by means of an upper cross member having means for fastening the upper fixation strap of the seat to the car; - a vertical support and a pneumatic cylinder, wherein the tilted frame is related to the rear section of the base frame by its upper frame by means of the vertical support, and wherein the pneumatic cylinder is disposed inside the vertical support and projecting from the vertical support at the top to the headrest support; - a frame by which the seat rest is supported, wherein the frame is mounted on a mechanism which determines the vertical movement of the seat rest by actuating on a side controller - components that constitute the support of the sides and of the headrest consisting of angular plates installed in a vertically sliding arrangement in the rigid frame, wherein the angular plate corresponding to the headrest presents a U-shaped constitution and the angular plates corresponding to the sides are formed from corresponding angular pieces (14 and 15), which are installed with the possibility of side movement, such that the headrest and the sides may be raised as the child grows and, further, the sides are also separated in a lateral movement in accordance with the child's growth; and - whose rigid frame receives the coupling of components that constitute the exterior support of the constituent parts of the seat, including said components a base, a sides (18, 19) and a headrest, whose parts are formed from blocks of pressure-absorbing lightweight material.