Stephanie Tombrello. child passenger safety specialist. You can safely switch your child to a booster seat if she's at least 4 years old and she weighs 40 pounds or more or has grown too tall for her car seat (when her shoulders are higher than the top set of harness-strap slots in the car seat's back).o matter when your child finally outgrows her car seat, it's imperative that you don't skip the booster-seat step and go straight to using seat belts alone. Although many parents do this (94 percent, according to recent reports), it's a dangerous mistake.