Most children when killed in a traffic accident were not correctly placed in the seat belt, car seat, or booster, or had let themselves out of the restraint. In fact, many had been riding completely unrestrained.
It is extremely important that all children under 12 always ride in the back seat. This was true even before the arrival of airbags, and is especially true now. Infants and young children should never be in the path of an airbag. In the backseat, the child is also afforded more distance before they hit anything hard, in the event of a crash.
Most states have child restraint laws, which specify the ways in which each age group should be restrained in a car. Unfortunately, many leave a gap for children aged 6-12: children who are too large for child safety seats and too small to fit into vehicle-equipped seatbelts. The best idea is a booster seat, which boosts the child up about four inches, enough for them to fit perfectly into the seatbelt. This is recommended until the child is large enough to fit comfortably and appropriately into an adult-sized chair and seatbelt.