When I was 6-7 years old, I had this book: https://www.amazon.com/You-Made-Million-David-Schwartz/dp/0688136346
I went out and found a copy of that book when my oldest was the same age. It explains everything at a reasonably good level for a kid. How to earn money, how savings accounts and checking accounts work. How loans work. How interest works, both for saving and for debt. Pretty much the main thing it doesn't cover much is investing. But anything else, it covers pretty well. Only complaint I would have now days is that the savings account examples assume a way better interest rate than we have available these days. It was more realistic when I was young.
Anyway, I wish this book was covered in 1st or 2nd grade in every school... And then maybe brought up again in 3rd or 4th when the interest will make a little more sense.