Id look for something harnessed that can be used later as a high back booster. I use a convert for my 3yo. I may find a bigger seat to keep him harnessed awhile though..he has one notch left and I still dont feel comfortable with no harness. I can still keep the seat he has now for later use though..
<blockquote><b>Quoting CerealMakerWife♥:</b>" I would go with a Graco Nautilus. It harnesses to 65lbs and can turn into a high back booster, then back less. She is way to tiny to not be harnessed."</blockquote>
and it's a booster until 100lbs.
Here they have to be 4 and 40 lbs to booster. I stopped harnessing DD about a month after she turned 4 even tho we gave the Nautilus. No matter how tight I got the straps (and I had DH crank them as tight as he could) she would get the clip down and pull her arms out EVERY time she was in that seat. Since we stopped harnessing she's been great. I sometimes have to threaten to pull the straps out but as soon as I do, she behaves