It would be hard to explain Bitcoin to children unless if they have a prior knowledge about economics. If you would explain the concept of Bitcoin to teenagers, just explain it in a simple and straight way. Explaining the whole concept might give confusion to the teenagers. If you want your child to learn about Bitcoin you can teach them first about investing, and if they have prior knowledge to economics, you can now teach them.
I also think that in fact children do not have much knowledge to support them to understand the relevant information of bitcoin and simply introduce them to the meaning of digital currency. For children living in the Internet age, they should have relatively high sensitivity to the digital economy degree.