Let them focus and school and sports and stuff.
Exactly kids should not be asked to earn bitcoin at this stage and its better to ask them to concentrate on their studies as there is entire life ahead to earn bitcoins.
Well somehow it is good to consider that kids are very naughty at all and they can use bitcoin for different transaction that they want to try.
But I think they must focus first in their studies and they must know about bitcoin yet, but if they are very knowledgeable tell them that bitcoin is simply as 'online money'.
As you said that kids are naughty so if we introduce them to bitcoins at this age then there is a chance that they may get diverted towards gambling, and if it happens then it can ruin their lives.
But I still felt that under 18-year-olds should know about gambling, about the odds of not winning and something about the psychology of gambling.