Apple – Support – Discussions – Smurfs app ….  So if you read this thread you will learn that some “Free” apps have some very costly in-game purchases.  The take away I would get is to restrict in app purchases as Meg St._Clair suggests.

Have you disabled in app purchases?

Settings>General>restrictions

Select enable then turn in app purchases to off. Unless you do so, you are authorizing those purchases. No password would be required.

Best of luck.