Well i think that the best thing to do is just let them know that bitcoin is a paying method but it is online only. Then maybe let them look at the forum and explore and see what happens with the market. Eventually they will understand.
I agree let them know that bitcoin exists. Sometime they will look it up and they will maybe find more information on their own. So after that they can also start to do something about bitcoin.
They can even create a new coin when they are young so they plan to make a coin in the future.