I don't think they accept anything but credit cards, so I don't suppose kids can use it at all. Which is even more sad in many ways.
When I was a 12, I asked my mom to let me use her credit card over the internet, (not for Sims paysites though, I've never bought anything from a paysite), and she let me use it. I assumed that was the case. But you're probably right, and it is sad.
PayPal is (or was?) another method of payment.
Doesn't PayPal only accept credit card transactions? I remember attempting to sign up for PayPal once, before I had a credit card, and it wouldn't let me because I didn't have one.