A child safety seat or a baby carrier for mounting on a motor vehicle seat, particularly a motor vehicle side seat, has a seat tray and a side impact bar mounted thereon which can be moved from a rest position located in particular within a standard width to a functional position located in particular outside thereof, and vice versa, wherein the side impact bar is positioned in particular on both sides of the seat tray so as to transmit any lateral forces behind the back of a child sitting in the child safety seat and to direct said lateral forces into the seat tray. A side impact bar is used with and mounted on the child safety seat or baby carrier.