I found DD2 (aged 7) taking money out of her money box today. She explained that she was giving it to DD1 (aged 10) in exchange for being allowed to borrow one of her Nintendo DS games. DD1 was charging her 25p for 10 minutes!
Obviously I made DD1 give back all the money (poor DD2 would have been bankrupt within a couple of hours) and explained that despite the apparently unlimited earning potential of her enterprise it was socially irresponsible in the extreme.
Not sure it sunk in though!