[wavefunction]

WoW apparently. As long as your kids don't have the means to purchase gold from Chinese gold farmers that should satisfy your criteria. They'll just be out hours of repetitive grinding or daily missions, which may induce the learning process you're looking for.

[Cpoll]

There's not a lot of scamming in WoW, since the GMs actively punish it. I imagine they'll even punish you for the "horse trading" scam if you do it blatantly enough.