“Most parents move their kids out of a booster seat before their child actually fits in a seat belt properly,” says Levine. And there’s no better place to see this mistake in action than in the carpool. “A lot of parents give in to peer pressure from other parents to take their kids out of the booster,” says Walker, because more kids can fit in a car without them. Kids might also perceive a booster as babyish, and be anxious to sit in the car like a grown-up. Walker warns parents to do the right thing for your child and keep them in the booster until the seat belt fits properly. And since seat belts are designed for people who are at least four feet and nine inches tall, that may not be until your kid is 10 to 12. But a seat belt that doesn’t fit properly can do more harm than good, piercing internal organs, damaging the spinal cord, or, if the shoulder strap is improperly fitted, seriously injuring the head.

The fix:

1. They should pass the 5-Step Test created by SafetyBeltSafe USA.

2. Even if a child is technically big enough, make sure he also has the maturity to stay seated in the belt in a safe way—not slouching, or putting the shoulder portion behind his back.