1st point:
You need to check the laws in your state. That will determine if your child weighs enough, or is tall enough, to be in a booster.
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2nd point:
We put K in a booster seat at her proper weight and she did fine. The only problem is making sure YOU plan ahead so your child will have enough time to buckle themselves. I take a child to day care and she is as helpless as they come. I refuse to buckle her because if I offer a reward for her to buckle she does it within a few seconds. She can do it herself but wants to be waited on. So I often site 10 minutes while I wait for this child to buckle. This childs parents do everything for her, even take her to the bathroom, and she is nearly 5 years old.
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Anyway, every child is different. Every state has different laws...some states say "6 yrs. old or 60 lbs" and some say "8 yrs. old or 80 lbs" for car seats/boosters. It really depends on how much longer you'll need the seat as to which one you purchase. The one we use now would work up to 60 lbs or even 80 lbs, but the one we have for J would not work for a child over 45 lbs.