Trout,
not to pile on, but good choice. these guys gave some great advice. I have made the mistake of thinking that kids bows have to be high performance. I have perry reflexed, bamboo backed, heat treated, etc. on kids bows. Yes, its nice for the bow to perform well for its size, and that is a concern because if a kid can't shoot accurately or get his arrow to even stick, he just gets frustrated. The other side of this coin, is the way most of us go. We are so concerned with performance that, we make a bow that we want them to be able to pull, not one that they really can. (adults do this all the time you would think we would learn...how many guys have bought a bow over 55 lbs, and quickly realized that 45-50 was what they really could shoot well) Why do this to a kid, they just want to have fun? besides, you get to build another bow when he's ready, bonus!
The point of all this is that, yes you can add a wood backing of some sort to raise the lbs. but be carefull, the dimensions of the kid bow are already exaggerated. I put on a 1/8th backing once and quickly realized how much i had just screwed up. then when i retillered to compensate, i had very little bow left. I think piking or heat treating is a better option, if you have your heart set on it...but i would just let him shoot it and then make another later.