EMERSON Article ~more details (booster seats)

BOOSTER SEATS

Many parents wonder when they can safely move their child to a booster seat. A booster seat can have a high or low back, and its sole purpose is to raise the child up to fit properly in the adult seatbelt. A booster seat does not restrain the child, but allows the seatbelt to restrain them. A booster seat may ONLY be used with a lap/shoulder belt system, NEVER with a lap belt alone!

You may not use a booster seat until your child is over 40lbs, and even then, you may want to consider keeping your child harnessed if they are not mature enough for a booster. It is not recommended that a child use a booster seat until they are at least 4-1/2 years of age.
To safely use a booster seat, your child must sit properly at all times. This means no napping, no wiggling/squirming, no reaching for toys, or slumping, or leaning. Any movement or shifting that puts the child out of position for the seatbelt to fit properly puts your child in danger of being injured in a collision.

If your child weighs at least 40lbs, and is able to sit properly in their booster seat every time, make sure they fit the booster properly. The shoulder belt needs to come over the child’s shoulder, not the neck, and should never be placed behind the child, or under their arm. The lap belt should come across the child’s hipbones, not their upper thighs, and NEVER over their abdomen. If the seatbelt does not fit properly, it will not protect your child, and you may want to reconsider keeping your child in a 5-point harness for a while longer.