Use booster seats until the safety belt fits properly, but at least to age 8 and 4'9" tall; use lap/shoulder safety belts once children outgrow booster seats.
When is a child ready for a safety belt?
The child can sit all the way back in the vehicle seat.
The child's knees are bent comfortably at the edge of the seat.
The shoulder belt is crossing the center of the chest and resting at the shoulder (not the neck).
The lap belt fits low and snug on the hip bones, touching the upper thighs (not the stomach).
The child is able to stay seated like this for the whole trip.
Make sure your child does not tuck the shoulder belt under her arm or behind her back. This leaves the upper body unprotected, putting your child at risk of severe injury in a crash.