A vehicle-mountable child safety seat assembly is disclosed. The seat assembly includes a safety seat and a prop assembly mounted with respect to the safety seat. The prop assembly has a stowed position and a safely deployed position. The prop assembly includes: a leg assembly; a foot assembly mounted to the leg assembly; and a first sensor, for example a tilt sensor, for sensing when the prop assembly is not in the stowed position. A second sensor for sensing when the prop assembly is not in the safely deployed position is also provided. A warning system is also disclosed. The warning system changes from a timing state to the warning state if the second sensor senses that the prop assembly is not in its safely deployed position by a pre-determined time after the change from a standby state to a timing state.