I think we can start explain bitcoin by tell them that bitcoin is like other international currencies dollar, euro and many else. They will understand and you explain more about bitcoin.
And also show them how easy it is for them to get involved, earn and start their own investment. Show them various ways to earn coins and give them a brief background about cryptocurrencies, make sure that you don't get too technical. I'm sure they will be more than willing to learn about bitcoin once they see it as an early opportunity to earn ample amount of money at a young age.