My son burned us for $100 with some stupid in-game app. He bought in-game currency with real-world money that I earned. Obviously he didn't know what he did until I explained it to him. He didn't spend any of the in-game currency, but I couldn't get the app developer to refund our money. The credit card company said that we could contest the charge, but that it would probably be decided against us (they were sympathetic though, for all the nothing that's worth).
The simple way to prevent this is to turn the tablet's internet connection off while the kids play games. I know that now.
The better solution is for us all to go outside and play.