Child safety seats are extremely effective when installed and used correctly. A child between 40 and 60 pounds (usually between the ages of 4 and 8) should use a booster seat, following the manufacturer’s instructions. Usually, a child weighing more than 80 pounds is big enough to wear a lap-shoulder belt. The shoulder strap needs to fit snugly. Never place a shoulder strap under a child’s arm or behind the neck. If the strap crosses the child’s face or throat, us a booster seat in addition to the safety belt.