Many parents are surprised to learn that safety belts generally do not fit children until they are between 8 and 12 years of age. Until that time, children who have outgrown a cars seat with a harness are safest in a booster seat.
Car seats and booster seats are designed to protect children and make them comfortable at the same time. There's nothing comfortable about a too-big safety belt cutting into a child's stomach or pressing against his face. Plus, kids who ride on booster seats can easily see many things they would otherwise miss.