Except drawing to far back on the whammo slingshot won't result in loosing a hand or face. Well, maybe loosing an eye or a serious welt when the surgical rubber tubing snaps. But you get my drift
If you make the point that adding powder is much the same effect then yes. When I show people how to reload I constantly remind them that attention to detail is paramount, and if your nephew is mature enough to safely reload, then it is great to be teaching him how to do it.