Test the belt that holds the car seat by pulling on the lap part of the belt after it has been attached to the car seat and notice if it lengthen easily. Push on the top of the car seat and see if the car seat is can easily rock or wobble. If the belt doesn't stay tight there is a safety clip that can be used to help the belt lock during a sudden crash. A booster chair can actually benefit a child usually to the age of nine. A child should move into a lap belt when he or she is more than the weight limit of the booster chair, when the child's shoulders are too broad, or his or her ears are above the back of the restraint, usually 40 pounds.
Whiplash can occur when the child's head sticks up above the seat back.
When considering to move your child in a lap belt check to see how the lap belt fits. If the belt fits low and tight across the top of the user's thighs the child should be able to sit comfortably and safely. The correct position to seat in the car is with ones buttocks against the seat back during the entire ride. Slouching, which can happen with children, does not allow the seat belt to fit properly. A shoulder belt is another seat belt that you might consider putting your child in after a booster chair.