The Browning Micro Midas IV is a wonderful childs bow. It is adjustable from 18 to 28 inches (in 1" increments) and is available in 30 and 40 pound models. Those are max weights, of course. The lightest you can get them is about 18 pounds....SUPER adjustable, very smooth. Just an awesome shooting bow.

The Hoyt Rintec is awesome as well. Same DL adjustments, and weight, but the cost is more. You can get the browning set up for about 270 and the Rintec is more for the bare bow......Couldn't go wrong with either one though.