If your child reaches 4'9" before age 8, he or she can legally ride in an adult seat belt. An exemption is also available for children who have a medical or physical condition that prevents them from being properly restrained according to the law. A written statement from a physician is required for this exemption.
Children who weigh more than 40 pounds can legally ride in a lap-only belt instead of a booster if that's all that is available in the vehicle. A lap-only belt without a booster is also legal for children weighing at least 40 pounds if all other seating positions with lap and shoulder belts are being used by other children. In this case, though, parents should try to rearrange the placement of car seats and booster seats to ensure that each child is protected as much as possible.