Really?
You're entering your CREDIT CARD details to authorise a payment. You might want to try reading the details and ensuring that you're fully aware. The terms and conditions make it clear that your details will be stored, and they also make it clear that by opening a children's account, they won't have permission to buy anything.
And that's not even mentioning the fact that every subsequent purchase on Live makes it clear that MONEY (not points) is going to be debited from the card.
It's not a failing on Microsoft's part (and it's rare that anyone would say that), it's a failing on the parents part. Genitals are far too easy to use, unfortunately, it's the followup that takes some common sense and intelligence.