Unscrupulous app developers are in the news again today, with parents suffering “bill shock” when the kids buy expensive in-game items without the parents’ permission. There’s a setting in iOS that makes such things possible even if you don’t share your password with your kids, and it’s a good idea to change it.
It’s in Settings > General > Restrictions > Allowed Content (I’m assuming you’ve already used Restrictions to set some parental controls; if not, you need to click Enable Restrictions in here). By default In-App Purchases are enabled, and where it says Require Password, the default is 15 minutes. What that means is that if you enter your password to download a free app, the password is then stored for fifteen minutes – and during that period apps can sell expensive in-app purchases without asking for the password. Some kid-targeted apps are well aware of that.
The simplest thing to do is to disable in-app purchases altogether, but if you don’t want to do that then you really ought to change the Require Password setting.