I think it's you who are confused.
You stated:
"i even got them debit cards so they could buy them online without bugging me to do it."
Presumably in response to the question in the op:
"Do you let your young children play video games with mature content?"
Thereby implying that you are perfectly happy with your children making their own decisions on which games are appropriate for them to play.
In order to point out the discrepancy where you trust your children to be mature enough to make that decision (and feel it is perfectly acceptable to ignore the law on the age restriction), I simply queried why stop at a debit card, and if/why you would feel it was inappropriate to provide them with a credit card?
It seems to have touched a nerve however, so apologies if I've upset you, as that wasn't my intention