Google implemented a PIN system into the Play Store app on Android. If you want to make a purchase, you have to enter a PIN. It’s a great idea to protect yourself from someone handling your phone, possibly planning to buy every game they can find.
However, that PIN is completely unsafe from anyone who knows how Android works. Yes, that means even your 12 year old can get past it easily. The PIN is stored locally, so going into Settings > Applications > All > Google Play Store and wiping data will remove the PIN. It’s that simple.
This (big) security hole has already been reported to Google, so let’s hope they make the PIN part of your Google account. But on the bright side, it’s safe from your toddler!