As you correctly point out, children's heights and weights do vary widely with ages. Children are at increased risk for airbag-related injuries because of their size. The vehicle seat belts, when worn, do not fit most children correctly. As a result, children often place the shoulder belt behind them which, combined with the child's short stature, places the child's face and neck directly in the path and full velocity of the deploying airbag. Thus, the safest place for infants and children to ride is the back seat of the vehicle.
Whether the vehicle has an airbag or not, children are reportedly up to 29 percent safer riding in the back seat as compared to the front seat. Even with limited crash data, the National Highway Traffic Safety Administration currently recommends placing all children in the rear seat. In contrast, the American Academy of Pediatrics advises that the most important factor is the size (height and weight) of the child to ensure proper fit of the seat belt. Remember, the lap/shoulder belt should be used as follows: position the shoulder belt across the chest, the lap belt snug across the thighs. Most importantly, any child riding in the front seat must be correctly restrained. Airbags and other child's transportation safety issues are evolving at a very rapid rate, so keep your eyes and ears open.