Cut the risk. The use of belt positioning booster seats have been shown to cut the risk of injuries by half. Fancy aftermarket neck pads or belt adjusters may increase a passenger's risk of injury. Early use of an adult belt is better than no restraint at all because it prevents ejection from the vehicle. However, severe injuries can result when the belt jumps over the pelvis & impacts the abdomen.
Yes. Children age four (4) through age eight (8), and measuring less than four feet nine inches (4'9") in height, must be secured in a belt-positioning booster seat system. This actually a law in tennessee.