It's frustrating because ads don't work well for the users and parents often hate in-app-purchases. Really the best way to monetize a kid's app is to charge for it up-front. But that makes it hard to market. You want to get free downloads to build momentum and fans but then you've got to feed your own family (which users never seem to understand). At least on iOS it's easy to schedule a "free for a day" sale to get attention. Amazon has "Test Drive", their in-browser free trial and NOOK has a separate APK upload for a free trial. (I haven't tried the NOOK free trial yet but I suspect that not many users know about it.)