I loooove this idea @lukestokes !
I really think " personal finance " or " money stuff related "should be teach in families and at school. Unfortunately, especially in Europe, nobody talks about money (this is why I started years ago blog/youtube about money education). People are ashamed to talk about money...
So, teaching money to kids : that's the best idea I have read today !
Your boy is already aware about money, and it's value (what he can buy, how it can grow). Having theses knowledges are making him powerful.
You are planting a seed through him, I am sure he will empower people / kids / friends around him soooon !!!