So if the animatronics are being controlled by the bodies/spirits of the children, then maybe, the killer of the children was a previous gaurd. That's why they kill security gaurds and no one else.
And before you pull the "they think you're an endoskeleton" card, why would dead children think that? I think the gaurd before you just inferred that after finding the gaurd(s) before him stuffed inside exoskeletons.