For children who have outgrown seats with a harness, use a belt-positioning booster with lap/shoulder belt in the car. It fits kids up to 80-100 pounds. Most kids need boosters from about age 3 or 4 to at least age 8. Children need head support; use a high-back booster if the car has a low backseat. A backless booster can be used if the car's seat has a headrest.