Dazed: Teen girl pleads guilty to stabbing friend for Slender Man.

Another reason to limit/moderate internet usage. My favorite way to convince parents: "Imagine a teacher told your child to write a report on the JFK assassination, and told them to research it on the internet. What do you think they would write?"

Beyond that, young children (in the absence of direct experience) tend to believe what they read/see. Even on television or smartphone.