In the first game there was a theory about five children getting killed by one of the employees that work at Freddy fazbear'pizza and were never found so the parents started to smell foul oder from the animatronics.So the parents started to realize the animatronics  had blood and mucus around their eyes and mouth and stopped bringing their children to the place.This is saying that the missing children were stuffed inside the suits and possessed them so they can get revenge on who ever killed them

Hey guys I know my opinions on the theories might suck but I am trying my best on them So no mean comment Bye Fnaf Fans