well kids like to play, so something that can bring together bitcoin and gaming
like puzzle that when solved reveal a private key for example
it is important that kids start to learn early some basic economy, at least something about income and expenditure
i heard about a new law for some countries that will make it possible(to teach them basic economy) in schools
before teaching first be specific. tell them the most easier way to understand to do with bitcoin. explain them house to use and how to manage bitcoin to the easier way.