i did a 48" kids bow for a buddy of mines boy. it was a 1/4 inch slat. i glued on an 8" riser and tapered it just a hair into the slat so that the riser wouldn't pop off. it was about 1 3/4" at fades and straight tapered to just under 1/2" tips. i backed with brown paper, camo painted, cut the shelf to center, and put in a greatly reduced locator grip, that i wrapped in bailing twine and TBIII. i was shooting for about 10-12 lbs for a 5 yr old. if you want 20 lbs, you can always add a thin backing strip, which will bring up the weight. tiller from the side when you start making bows that thin.
don't forget that backing is a safety factor, for the abuse a kid is sure to inflict on a bow...even he treats it right, one of his friends is sure to pick it up and over draw it.