A child's safety seat (10) is for use in a land vehicle, air or marine craft having an adult seat (31) equiped with a safety belt having a lap strap (32) and a shoulder strap (34). The safety seat (10) comprises a seat body having a seat portion (12) and a back portion (14) supported on a frame having a pair of side members (20, 22) each of which has a back end located behind the back portion (14) and above the seat portion (12) and a front end located in front of the back portion (14) and below the seat portion (12), each side member (20, 22) being shaped so as to have an intermediate portion between its ends located behind the seat so as to engage with a lap strap (32) of an adult safety belt whereby the frame is securable to the adult seat - (31). Guide means (40) for a shoulder strap (34) of the adult safety belt are secured to the frame seat and adapted to allow the shoulder belt (34) to pass therethrough in one direction but to inhibit movement of the shoulder belt (34) therethrough in the opposite direction.