You could do that, but it seems kind of obvious and condescending? Cause right now you can forbid it and theyll see all their friends lose money and never win anything until your child is of legal age. For everyone else, it seems like some kind of inherited cultural addiction that gets culturally passed down. Just openly mock gambling as evil/a scam/rigged and they'll probably be more likely to pick that up if you back with principals instead of taking the weak, confusing, and contradicting stance of feeding $50 into the evil system.

My parents did something similar for me, and I still remember the lesson a few decades later.

I think the details matter here. If you give your child $10 every month to make one bet, sometimes they win, sometimes they lose; you are doing it wrong. If you give them $50 and let them make a series of bets not exceeding $10, they will learn that the curve sometimes goes up, sometimes goes down, but ultimately ends up at zero. Not because "you were not lucky today, but maybe the next time", but because this is how the system works.