A booster seat for children, comprising: - a frame composed of a front structure and a rear structure, said front structure being composed of a lower base portion and an upper backrest portion comprising two upper lateral uprights (7a, 7b), said rear structure being rotatably attached to the lower base portion; - a seat portion rotatably attached to the upper backrest portion; - a tray comprising a base plate and a pair of lateral arms, which are connected to the base plate; - engagement/disengagement means for reversibly attaching the tray to the upper backrest portion of the booster seat, said engagement/disengagement means being composed of a pair of first components respectively fixed to the upper lateral uprights (7a, 7b) and a pair of second components which can be reversibly engaged with the pair of first components respectively; characterized in that - the pair of second components is rotatably attached to the pair of lateral arms of the tray to commute said tray between a use configuration in which the tray overhangs the upper backrest portion and a stored configuration in which the tray is substantially parallel to the upper backrest portion; - the booster seat can be commuted between an open configuration and a closed configuration whereby, in the open configuration, the lower ends of the lower base portion and the lower ends of the rear structure are spaced apart with the seat portion substantially parallel to the tray which is in the use configuration, in the closed configuration the lower ends of the lower base portion are close to the lower ends of the rear structure with the seat portion and the tray substantially parallel to the upper backrest portion. - a frame composed of a front structure and a rear structure, said front structure being composed of a lower base portion and an upper backrest portion comprising two upper lateral uprights (7a, 7b), said rear structure being rotatably attached to the lower base portion; - a seat portion rotatably attached to the upper backrest portion; - a tray comprising a base plate and a pair of lateral arms, which are connected to the base plate; - engagement/disengagement means for reversibly attaching the tray to the upper backrest portion of the booster seat, said engagement/disengagement means being composed of a pair of first components respectively fixed to the upper lateral uprights (7a, 7b) and a pair of second components which can be reversibly engaged with the pair of first components respectively; - the pair of second components is rotatably attached to the pair of lateral arms of the tray to commute said tray between a use configuration in which the tray overhangs the upper backrest portion and a stored configuration in which the tray is substantially parallel to the upper backrest portion; - the booster seat can be commuted between an open configuration and a closed configuration whereby, in the open configuration, the lower ends of the lower base portion and the lower ends of the rear structure are spaced apart with the seat portion substantially parallel to the tray which is in the use configuration, in the closed configuration the lower ends of the lower base portion are close to the lower ends of the rear structure with the seat portion and the tray substantially parallel to the upper backrest portion.