Make it as long as he is tall, and half as wide as your bow. Like if you make a red oak bow 2" wide to mid-limb, make his 1" wide.
You will find that making a kid's bow is as challenging as making your own. The key is making it narrow enough to begin with, if it is wide then you need to make it so thin that a tiny amount of taper makes a big difference. 1" or even a little more narrow is a good idea. As for poundage, it'll be low and I don't know that I'd measure it, just have him try and pull it every so often and keep thinning it until he can pull it back.
I agree that you can hunt with a board bow.
As for backing, its only necessary if you don't have a real good board. If your grain runs straight on all four sides, with few runoffs, you can make a selfbow with no problem and it will perform a little better than a backed bow.
Have you been to Jawge's or Ferret's site? They have great instructions on selecting boards and making board bows.