Abstract: A child seat device, for use in a vehicle having a seatbelt for securing the child seat device to the vehicle seat, has an inflatable child seat which includes an inflatable bottom panel, an inflatable back panel upstanding from the rear of the bottom panel, and a pair of opposed, laterally spaced inflatable sidewall panels. Each of the sidewall panels has a back portion projecting forwardly from a respective side of the back panel and a bottom portion projecting upwardly from a respective side of the bottom panel. An inflatable brace, disposed in each of the sidewall panels intermediate the back and bottom sidewall portions, limits pivotal movement of the back and bottom sidewall portions towards each other. Each of the braces defines a single interior air chamber having at most limited gaseous communication with the bottom, back and sidewall panels.