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
In today's generation kids is already knowledgeable about technology so you can easily teach them about bitcoin give them the knowledge you know about in bitcoin and they an already understand you because kids ca learn easily when it is related to technology. But if you are going to teach kid talk to them slowly so they can understand you.