She is petite! My daughter was a little shorter, but even she was heavier at five years old. LOL She was 32 pounds at her five year appointment, and I forget how many inches. 40, I think.
What seat is she in? A convertible goes from rear to forward facing. A combination goes from harness to booster. If she's in a convertible that turns into a booster then it's a 3 in 1, and it's possible her shoulders are over the top usable harness position anyway. It also helps us narrow down what to recommend to know the exact seat she's in. If it's an Evenflo Triumph Advance, for instance, then we don't want to recommend that one to you.
The first thing that comes to mind, though, is the Graco Nautilus. It fits small kids very well, and turns into a very decent booster. It's what my daughter rides in, and she's still harnessed at nearly seven years old and 40 pounds (it has a 65 pound harness limit, not that you'd care, since your daughter will never hit 65 pounds harnessed. Neither will mine). She's probably got another six months tops in the harness before her shoulders go over. Then she's got it as a booster for years to come.
Wendy