Scroll down in this section to a few other discussions about trailer bikes. Last year when I was shopping, my very petite 4 year old could not even come close to reaching the pedals on the Adams. I found that the Trek Mountaintrain had a shorter seat tube by several inches. Still, I had to make two mods to capitalize on the lower frame so that she could pedal.
1) Get a shorter seatpost (you could cut a few inches off the included one, but I just couldn't do it!).
2) Invert the clamping hardware under the seat itself to drop another half inch or so.
With these simple changes, she will have years of potential use.