Booster seats can improve the fit of adult belts for children who have outgrown child restraints, but not all boosters provide the recommended belt fit. It's important to ensure that the lap belt fits low across the upper thigh, not across the child's soft abdomen. The shoulder belt should cross snuggly over the center of the shoulder.
When older children start using the vehicle belts, the shoulder belt should rest across the chest, away from the neck, and the lap belt should fit low and snug across the upper thigh. Your child needs to sit straight up with knees bent at the edge of the seat.