Consumerist (our sister site) just ran a story about a couple of hapless parents, a five-year-old Littlest Pet Shop player, and a $120 charge the child incurred making in-app purchases for the game. If you're concerned about the same thing happening to you, here's how you can prevent it.
First, Apple's policy on in-app purchases (which can be bonus game maps and levels, items to advance you faster within a game, or weekly or monthly subscriptions, for example) is that you have to enter your iTunes Store password to make the purchase. If you're using an Apple device that's running iOS 4.3 or later, you must tap Buy each time you purchase something within an app—unless fewer than 15 minutes have passed since your last purchase, in which case you don't need to retype your password.
This is where it can get tricky: If you enter your password for your child and walk away, the child can just keep buying and buying, as long as 15 minutes don't elapse between purchases. That can add up quickly.
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You do have a couple ways to prevent this, though. Using Apple's parental controls, you can block access to specific content types, including in-app purchases. You can also adjust the time required to pass before a password is needed to purchase content—all the way down to zero, as the parents did in Consumerist's story.
If your child still manages to rack up a hefty bill, you'll need to appeal to Apple. The family managed to get store credit for most of the $120 charge they received.
We checked the Google Play Store's policy on in-app purchases as well, for those using Android devices. Simply put, "all refunds are at the discretion of the developer" (the app's developer, that is). But before you get to that point, you can set a PIN that has to be entered before any in-app purchase can be made.
On Windows 8 phone devices, you can set up a child profile that will allow your child to download only free apps and games—which also prevents any in-app purchases, even from within free games.