I've got to turn on the Parental Controls the next time I let the kidlets near the Fire HDX. The 6 year old bought the video "The Biggest Machines In The World", when the video he had been previously watching ended. He told me it cost $2.99 but apparently that is the rental cost - and he hit the "buy" button instead ($9.95).
That'll teach me to take a bathroom break while that device is in his hands!