Am I missing something? When they went to add money, do you go back in and increase the initial investment?

Are they actually investing anything? If so, wouldn’t the app for the brokerage do this with real numbers?

They invest with “The first national bank of dad” as user loloquwowndueo pointed out, I'm their broker, no penalties when they want to make a withdrawal

I think what he's saying is: Given a balance of $50, they "earn" $5/mo. Given a balance of $200, they "earn" $10/mo (or $199*0.10 + $1*0.05). If they don't spend it, I'm assuming it's "rollover", and if they eat into their capital (eg: buy an xbox) then they feel the sting of earning less-per-month.