Vehicle child seat with means for tightening a safety belt

The seat has an activation unit (2) that acts on a mobile unit (1) so as to move the unit (1) between a release position and a tensioning position for increasing the tension of a seat strap. The activation and mobile units are mounted with respect to each other in such a way to lock the activation unit in the tensioning position. The activation unit is locked by a force exerted by the strap on the mobile unit.