The invention concerns a child seat, comprising a base and a back, said back having two back elements, respectively corresponding to said back left-hand side and right-hand side (11, 12), partially overlapping, at the back central part, and mobile relatively to each other, symmetrically with respect to a vertical axis, corresponding to the back centre, so as to enable said back to be adjusted in width, between two extreme positions. Advantageously, it consists of an adjustable headrest (15) simultaneously enabling to adjust the seat in width.