One point a lot of you are making is that the parents shouldn't have given the kids access to their credit card but I suspect you don't know how this stuff works.
Now, my experience is mainly with Apple i-whatever stuff as that's what schools mainly use, so I don't know if this applies to Android stuff too, but with Apple stuff it is trivially easy for a kid to run up charges inadvertently with the default settings on an iPad or iPhone.
The problem is that most people use a credit card to buy stuff from the AppStore rather than using vouchers, and once you do that the card is registered to your AppleID and any purchases *automatically* get charged to your card!!
The ONLY barrier is your AppleID login and by default i-devices only ask for it once, and once you've logged in it caches the ID until you clear it. The kicker is that loads of stuff requires you to be logged in to your AppleID to even work.
Because this happened to so many people in the US (Where people are a lot more litigious), Apple added an option (IIRC iOS6+) that, when enabled, means you have to type in the AppleID every time it's required instead of just once and caching it indefinitely.
This is a bit of a PITA as it turns out many things require you to type in the AppleID multiple times, but this sheds light on why they were caching it in the first place.
However, it is, by default, not set, so the owner of the device would have to know about it in order to take advantage of it.
It's also been said that if parents were supervising their kids when doing this stuff (Which, I agree, they should be, for that sort of age) they would have spotted this, but some of these games are dead sneaky and disguise the purchases so it's not immediately obvious whether you're buying something in-game or for real, esp. since as mentioned above they can just charge to your card so seamlessly.
The problem is most of these devices are easy to use so they are bought by noobs which makes them an easy target for this sort of grey-legal fraud.
It's just a symptom of the commoditization of computers into mainstream appliances for people who won't/can't spend the time to learn and it's not going to get better any time soon until we're locked out of our own machines like we are with modern cars!
Actually, this is a good life lesson in general: Don't let ANYONE or ANYTHING save your credit card details. (Or any details for that matter...!).
I got bitten by this by my insurance company on my first renewal (It turns out unless you tick the tiny box that says don't save my card details they auto-renew your policy unless you phone them and tell them not to!); Got charged a £10 'admin' fee to terminated it, but saved £100 transferring to a different company so it wasn't a total kick in the balls...