It's not a real person. It's a made up fictional person the children were trying to impress. The site is for fictional writers, although the content gets moved around the internet often.
However, at the heart of it, the children were only pretending to be friends with the girl they were planning to kill, simple as that.
People are trying to blame this on a fictional character, but, IMO, the parents are to blame for not instilling, after 12 years, the worth of another person's life to their child. This isn't an internet issue. And, there is no way, this is the only warning sign these parents got, not by a long shot.