My kids have a shared iTunes account. They upload iTunes gift cards to is. That is the only form of payment connected to the account. I've explained the gem and coin thing to them with the games that they have. If they want to waste the typical $10 gift card they get for birthdays and such, then they are free to do that. I've warned them of the purchase of gems/coins just to get stupid stuff in their games.
I think it is foolish to attach an actual bank account to iTunes when it is much easier and safer for your account just to buy a gift card, even for us adults. I don't even like buying things online and I think I've only bought from
Amazon and
Walmart, I surely am not going to put CC info into iTunes, especially my kids' iTunes.