A booster seat (1) for children comprising a frame (2) composed of a front structure (3) and a rear structure (4), the front structure (3) comprising a lower base portion (5) and an upper backrest portion (6), the rear structure (4) being rotatably attached to the lower base portion (5); a seat portion (8) rotatably attached to the upper backrest portion (6); a tray (9) comprising a pair of lateral arms (11); the tray (9) being rotatably attached to the upper backrest portion (6) for the tray (9) to be commuted between a use configuration in which the tray (9) overhangs the upper backrest portion (6) and a stored configuration in which the tray (9) is substantially parallel to the upper backrest portion (6); the booster seat (1) being adapted to be commuted between an open configuration and a closed configuration whereby, in the open configuration, the lower ends of the lower base portion (5) and the lower ends of the rear structure (4) are spaced apart from each other, with the seat portion (8) substantially parallel to the tray (9) which is in the use configuration, and in the closed configuration, the lower ends of the lower base portion (5) are close to the lower ends of the rear structure (4) with the seat portion (8) and the tray (9) substantially parallel to the upper backrest portion (6).