Please, PLEASE do some research into your state's child passanger safety laws.
Seat belts should NEVER be shared.
As for booster seats, most state's laws have not caught up with recognized risks and therefore, even if you are compliant with the law, you may be putting a child at risk.
Most kids, even those who weigh over 40 lbs, really need to be in a booster seat in order to properly position the belt over the hip bones.
You are not really "big enough" for an adult belt until you can sit properly with your feet on the floor and the belt on your chest (not neck).
Furth