Why not use board game Monopoly as a comparison? Cryptocurrency sort of money which you can use for particular "game". This might work for children. They do not need to dig deeper into technological stuff. ![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
But why do you want to explain Bitcoin for children?![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
They do not need to know any knowledge about a cryptocurrency has nature against all rule of the world's economy.
Understand it soon, they can have wrong optinion when they still young and someday, they can become the terrorists.
And they won't know what to do with the information you give them.