Well speaking from experience, building a kids gun from quality parts is a waste of time, and parts. I bought a cut down T/C Hawken in .45 for my nephews to shoot. They loved it, as did a couple on the other side of the family. Then my wife and I had a boy of our own. He shot it for a couple of years. But, I had drifted into shooting a trade gun, so of course he wanted a trade gun too. I built him one from a .410 shotgun barrel, a walnut stock, and a pistol sized flintlock. It shot fine but he outgrew it in no time.
The time, and money involved hardly justifies the short amount of time they use them. It’s much easier to cut down a used kit gun, than build a gun from scratch. Believe me it won’t fit them long.
Hungry Horse