Reader Question: My 3-year-old seems tall for his car seat. Can I move him to a booster now?
A: No, not if you can safely keep your child in a harnessed seat for a while longer. Most 3-year-olds are not ready to ride in a booster seat in the car. I suggest that parents keep their children in a harnessed car seat to at least 40 pounds and 4 years, but preferably longer. Today there are many car seats available that harness to 65 or even 80 pounds. Most children can fit into one of these bigger car seats well past age 4. My own daughter recently outgrew her 65-pound limit seat by height, though she only weighs about 45 pounds. She is 6.
Any step up in car seats - from rear-facing to forward-facing, from harness to booster - is actually a step down in safety. The 5-point harness spreads crash forces over more points on a child's body, lessening the potential force any one part of the body must take in a crash. If your child's harnessed car seat is used with a top tether, he or she can benefit from a reduction in head excursion during a crash, which translates to fewer and less severe head and neck injuries.
From a practical standpoint, parents tend to have an easier time keeping their child in the harnessed car seat altogether. The vehicle lap/shoulder belt is much easier to unbuckle than a harness buckle, so if your child is an escape artist, the problem may grow after moving into a booster. More importantly, the child must be able to sit in the proper position and be able to stay there in order to be safe in a booster seat. This means no leaning forward, sideways, slouching, or wiggling out of the shoulder portion of the seatbelt. The seatbelt cannot protect a child who is not in the proper position. Most children cannot be trusted to sit properly until at least 4 years old. Many parents find that their child is actually much older than 4 before they can be expected to sit still in a booster.
If your vehicle has lap-only seatbelts in the rear seats, I highly recommend that you keep your child in a harnessed car seat as long as possible. Harnessed seats can be installed with a lap-only belt. Booster seats absolutely must be used with a lap/shoulder belt. Extended harnessing is vastly preferable to moving a child into a lap-only seatbelt.
So, if you think your toddler is outgrowing his or her harnessed car seat, first be sure that you're checking the right signs to judge the fit. Pay close attention to the weight limits of the car seat and be sure you're looking at the forward-facing harness weight limit, not the booster weight limit (if it is a combination car seat). When your child is forward-facing, the harness slots should be at or above the child's shoulders. When the shoulders are above the top slots, it's time to change seats. A forward-facing car seat is also outgrown by height when the tops of the child's ears reach the top of the car seat shell. Most children outgrow harnessed car seats by height long before they outgrow by weight, particularly with the 65-pound seats.
If your child truly has outgrown a 40-pound limit harnessed car seat and is still under age 4, I suggest looking for a car seat with a higher harnessed weight limit first. There are a few seats available today with a higher harness limit that later become booster seats, if you're concerned about buying another car seat and then a booster. As long as it is used properly, any car seat with a higher harnessed weight limit would be a good choice.
Heather Corley is a certified Child Passenger Safety Technician.