Firstly of all let’s treat bitcoin as a currency first, as currency then it is fair to expose children to it because with this they will learn just another means of making payments not using fiat.
Bitcoin has many utilities and a mean of value exchange, or you can call as a currency, is one of its utility. It's can not be denied but the world is still far to a time when you can use bitcoin as mean of exchange for everything in your life. There are things you will have to purchase, pay with fiat currency in either cash or bank transfer. So there will be time you need to exchange your bitcoin to fiat currency and do those payments.
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To me it is a new technology and should be taught nothing more about it.
There are some books, videos, games to teach children about bitcoin.