i think bitcoin is much easier to understand then fiat
a toy for to help kids understand bitcoin would be a piggy bank with a bitcoin logo on it.
money in, money out
try explaining to a kid what happens when you deposit money in a bank....
bitcoin is probably closer to what people BELIEVE money is, then what fiat money actually is.
I always found there to be similarities. Explain what Fiat is to you in your own words perhaps?