The only solutions right now are.... (not really pretty)
1) Pin lock your phone and don't give it to anyone
2) If it's your child's device, create a child account (problem is they can just hard reset and put in a new account and bill it right to your wireless bill...)
3) Kill the data (either block data on the device (long shot since it's a smartphone and carriers make it mandatory to have data plans), or turn off wifi and cellular data and hope the person using it can't navigate their way to switch em on)
4) Call ATT and have them put a block on ATT purchases (untested, see below for ATT's information)
I think if you're going to give it to a kid to play games installed, killing the data through settings is the most effective. If they're old enough to understand how to switch it on, they should be old enough to comprehend what accidental purchases are, and should be able to follow directions not to do it. After using Windows Phone, if I were to choose devices for a family, Windows Phone definately is not in the best interest.
I was going to do this to prevent accidental purchases, but it requires you to "donate" 50 cents to prove you're an adult using your credit card... I don't mind doing that, but I cannot see how that proves anything and why they have to charge you for verification. Yeah, sure its cheaper than getting charged up the wazoo in accidnetal charges, but seriously, everyone else can just put a password on the account to make things easier, and we have to pay for some cumbersome work around... and as I pointed above, a kid with access to the internet could just figure out how to hard reset, make a new account with a fake age (since you apparently don't need a $0.50 verification to make an account saying you're an adult), and input that new account on their phone and get whatever billed right to their parents' wireless bill.
EDIT: Stumbled upon this:
http://www.att.net/smartcontrols-WirelessParentalControls
Looks like you might be able to get a "Purchase Block," to prevent direct billing. That would mean kids would have to have a credit card on file to purchases things since they shouldn't be able to bill directly. This is something that boggles me again, because it is very similar to how carriers handled data before making data plans mandatory; people would be put on pay as you go data and then get bill shock. It's somewhat the same, but instead of little increments, it can happen sporadically.
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