Re: newb in Ct
If I understand your question right, then changing hubs is a good deal more difficult than just buying a new wheel. You'll have to dismantle the entire wheel; spokes, rims, etc. Then you'll have to re-lace an entire wheel.
Not that it's all that hard to do. But a newbie should not expect to get it done quickly and properly. It's the sort of thing that takes practice.
Maybe you should consider using your wheels as they are and just put plain old rim brakes on them?
And I'm sorry to be a drag here, but.....is it a good idea to give a motored bike to a twelve year old. You might be asking for a world of trouble there. But what do I know? This might be a particularly sober twelve year old.
Still, I'd tread very carefully on this.