When a child's shoulders are above the top set of strap slots, it is time for a booster seat. Booster seats protect the child's upper body with either the shoulder belt or with a shield. Most 40-pound children are not big enough to fit lap and shoulder belts properly. A belt that rides up on the tummy could cause serious injury. Many young children do not sit still enough or straight enough to keep lap belts low across their thighs.
Boosters are comfortable for children because they allow their legs to bend normally.
This also reduces slouching, one cause of poor lap belt fit. Because raising the child improves belt fit, these are called belt-positioning boosters. Some have a high back that gives head support for taller children. Some boosters of this type are built into vehicle seats. 2.Boosters with removable shields.