I don't know, but their victims didn't die from the knife wounds. http://www.washingtontimes.com/news/2014/jun/3/girls-12-accused-stabbing-friend-19-times-impress-/ http://www.washingtontimes.com/news/2014/jun/9/13-year-old-ohio-girl-stabs-mother-2nd-slenderman-/
The thing that scares me is that knives are actually much more dangerous. They're completely silent. If someone knows what they're doing, they can inflict a ton of damage without making a sound. Guns are obviously much easier to use, and can inflict a lot of damage in a short period of time, but they're loud. They raise alarms. If someone took their time and did it quietly, they could kill a lot of people before anyone even knew what was happening.
DING DING DING! You think those 14 year old kids know what they're doing? HELL NO! That's why they use their killmachines. It's way easier to kill someone with a gun than a knife at the age of 14.