Well you're still flawed if you scale your argument. What if you take your argument, and scaled it up. What if you promised each kid 1 trillion burgers. Will you have access to 1 trillion burgers tomorrow? What if they take their future 1 trillion burgers and sell half. What if you walk into McDonalds to claim the 1 trillion burgers. Does McDonalds have 1 trillion burgers? No.
So you're saying it's okay to promise burgers as long as it's an amount that actually exists and McDonald's can fulfil it. So what you're saying is that you shouldn't sell things you can't possibly fulfil? Hence the argument against this kind of trading.