Motor vehicle crashes are the leading cause of death among children ages 4 to 14. Car booster seats have proved effective at preventing serious injuries to children. Yet 86 percent of American children who should be restrained in car seats or belt-positioning booster seats are still inappropriately restrained in adult seat belts. The National Highway Traffic Safety Administration recommends belt-positioning booster seats for children who have outgrown their forward-facing car seats but don't yet fit into adult seat belts. Most children won't fit into the adult belts until they're at least 4 feet, 9 inches tall.