The way to explain something to a person is to do in terms they understand, you cannot explain quantum physics in the terms used by physicist to the average person you need to use examples they can understand, and the same applies to children you need to explain to them in terms they can understand.
Yes, for teaching bitcoins to a kid, I believe we must turn as a kid so that they could easily understand by being "within their world". I mean to say we need to enter into kid's world so that we could understand their terms and in playful way we could explain them what we really finds that they could understand.
First we need to classify what are easily understandable for a kid. Because we cannot expect them to understand complex things. When we are teaching them in their way, I believe you can find good results.