I feel like Bitcoin should first be introduced to teenagers and not to children. The reason is that they need to learn a couple of things about money in general first.

What children and teenagers need to learn first before Bitcoin:
1. Handling their own money
2. Financial management
3. Banks
4. Risk management

But if you just wanna explain Bitcoin as a term, you can just say they are like money but digital money. Like the simoleons in Sims or like neopoints from Neopets.