This story sucks.
The two 12yr olds should NOT be tried as adults and put in adult jail. They obviously have mental issues and should be placed in a mental health institute. Prison for a 12yr old will only kill them or make them worse humans. This type of kid being tried as adult crap happens way too often.
OP: But WTF? Stupidest kids of the year if you ask me.
I'd say It's the parents fault, but not for the reason most people would think. I seriously doubt their parents gave them the impression that killing someone is okay, but they failed to teach them common sense, and that bad actions have bad consequences.
I think some parents over reading too far into this though. Just because your kid knows who Slender Man is, it doesn't mean they believe he actually exist and will do evil things for him. As long as you ask your kids if they think he's real (If they say yes tell them that he's a creation of the Devil or something), and they don't have any mental issues, then you're fine.