After thinking about this a little bit I think I will probably wait until they are in the mid-teenage years until you teach them the importance of security measures. It is true that people will try to steal your bitcoin if they have the opportunity, however this is really not something you want to be teaching your innocent child.
At the same time, I would look out for their maturity. The maturity of one boy could be different from another boy. I would teach them the concept of money, stocks, and investing because introducing them to bitcoin.