After the toddler seat is the booster seat, which just puts the child up higher in the seat so the seat belt fits them properly. The best thing to do is keep them in a harness until they outgrow the weight limits of the seat; if you're still using a convertible seat now, they make toddler/booster combos that usually have a higher weight limit for the harness, it keeps them safer. Legally, at least in my state, kids have to be in a car seat (a backless booster is fine for later years) until they're 4'9" and (I think) 80 lbs, whatever age that happens to be, that's when a seat belt fits them the right way.
at 10:35 PM on Jun. 13, 2009