Once your child reaches 4 years of age and 40 pounds, he is too big for many toddler car seats. However, that does not mean he is ready to use the car's seat belt alone, as he is too small for the seat belt to fit him properly and keep him safe. A booster seat will lift him to a position to wear the seat belt properly; there are three kinds from which to choose.
High-back Booster Seats
The National Highway Traffic Safety Administration, or NHTSA, warns that if your car does not have headrests in the back seat, you must use a high-back booster seat for your child. This type of booster seat will protect your child's neck and head in the event of a car accident, as a built-in headrest in your car would.
Backless Booster Seat
If your car does have built-in headrests in the back seat, you may use a backless booster seat, also called a low-back booster seat. The Massachusetts Executive Office of Public Safety and Security advises positioning your child in a backless booster seat so that the center of her head aligns with the center of the car's headrest.
Convertible Booster Seats
The NHTSA advises that you may use a convertible booster seat from the time your child is ready to sit in a forward-facing toddler car seat, which is after he reaches his first birthday and weighs at least 20 pounds. A convertible booster seat features a five-point harness for toddlers, just as standard forward-facing toddler car seats do.
Once your child reaches the upper weight limit of the toddler phase of the convertible car seat, usually after he is 4 years old and weighs 40 pounds, convert the car seat into a booster seat by removing the five-point harness. The booster seat phase of a convertible booster seat features a high back, so you may use this type in a car with no built-in headrests in the back seat.
Booster Accessory: Universal Harness
Each type of booster seat raises your child to position the shoulder and lap seat belts properly on her body, explains the NHTSA. If your car only has lap belts in the back seat, the seat belt will not properly secure your child in the booster seat.
The Kentucky State Police recommend the use of the 86Y Universal Harness along with your booster seat if your car does not have shoulder and lap belts in the back seat. This harness consists of two shoulder straps and a chest clip. The tops of the shoulder straps come together behind your child into a long tether that you must mount to a factory anchor point in your car. You can find the anchor point in your car by consulting your owner's manual. The lap seat belt threads through the bottoms of the shoulder straps in front of your child at hip level when she is in her booster seat.
