OP: I think introducing volatility (which you can just model with one percentage variable and a sinusoidal multiplier based on this: 100% volatility means if your "real" balance was going to be $100 today, it varies between $0 and $200 ; 10% volatility means you're fluctuating between $90 and $110) is also a good idea in teaching kids to refrain from the impulse of withdrawing the money just because today's daily gain is in the red.

Another add to the feature request list :)