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Easily illustrated for children: https://www.youtube.com/watch?v=H4XL8s1BEdk

A book I read well over 30 years ago, and is still apparently updated today, is "The Kids' Money Book" by Neale S. Godfrey, which you can certainly find secondhand for cheap. There are updated versions authored by Jamie Kyle McGillian, and so long as they are in the original vein they are probably excellent.

However it's not something I would throw at a 6-year old. A sharp 8-year old should be able to grok all of it though.

Reminds me of this excellent video https://youtu.be/HLIJkmy3vy8 it's in Spanish, he tells the story of explaining a financial system crash to his daughter. Probably has subtitles in English, I recommend it

I was hoping most people would be familiar with the exponential chessboard story, of which this is pretty much the inverse.

https://en.wikipedia.org/wiki/Wheat_and_chessboard_problem

You need to understand exponentials to really get it. Where I live, they only teach it in advanced math classes in 11th grade, so elementary school seems ambitious.