|You should use a toddler/convertible safety seat in the back seat until your child outgrows it when she is about 40lbs. You can then use a booster seat until your car's lap and shoulder belts fit correctly (when your child weighs 60lbs to 80lbs and is 52 inches tall).
Many parents make the mistake of moving their child into their car's seat belts too early. Your child will not be ready for seat belts until the lap belt fits across her lap and not against her abdomen (in an accident, an incorrectly fitted lap belt will crush your child's abdomen). Also, the shoulder harness should fit across her chest, and not across her neck, where it would cause serious injury in an accident. You should also not place the shoulder harness behind your child, as it will provide no protection in this position.