Child Killer

The so called "Child Killer" in FNaF is the person who murdered 5 kids at Freddy Fazbears Pizza before you take the job as a security guard. A popular theory is that the Child Killer lured the 5 kids in the back by dressing up in a Golden Bonnie suit so they would trust him and later murdering them. This is known as the "Missing Children Incident", and in FNaF 1 you can see posters on the wall at the East Hall written about it while in FNaF 3 the phone guy says that the golden Bonnie suit had been moved.