The 6 Lost Ones in 1987

About
This event, coined by Bryan Renolds as The 6 Lost Ones in 1987, is an event that we know happened at Toni's Funhouse and Games, and seems to be the event that set everything afterward into motion. In a nutshell, The event happened in Toni's Funhouse and Games, 1 week after Freddy fazbears closed. A woman named Isabelle lures 6 different children to the back, rapes, and then murders them, all for the fun of it. Shorty after the diner closes, Michelle goes into the back and finds the horror's. Stanford also sees the horrors, and calls the police. Isabelle then comes in and says "Oh, call the cops, they won’t find anything." implying she hid or discarded of the kids.

Deaths
The first to go was Sarah and Shelia, the kids who would go on the possess Toni and Wendy. They were fighting about having fun and being laid back, when Isabelle came. She says she has something to make them stop fighting, and they go along with her. She then rapes and kills them.

The next kid was Tim, the youngest who would go the possess the Plusha tronics, but mainly Cody. he was lured to the back, and his head was split open.

After Tim was Jake, the kid to possess Kong. He was lured to the back, and tied up. Isabelle left him there for a while, before coming back. Jake tried desperately to get out, but couldn't. Isabelle then raped him, and killed him.

Payton was next, the one to possess pony Girl,. She was lured to the back, and after seeing everything, tried to get out. Isabelle then grabs her, and punches the knife into her neck, and comforts Payton until she dies.

The last kid was Audire, the one to become White Tiger. She was lured to the back, and according to her word, was "played with", and torched for hours on end, before dying the most gruesome death of the 6.

In the FNAT Universe
Judging by Ryan's posts, its clear this is very much an unsolved mystery, and one that seemed to not have been solved. It is also brought up by the animatronics multiple time's, taunting Elly about her crimes (since they mistake her for Isabelle)

trivia

 * This is the only known group of kids to die in this series, and the only animatronics to be possessed.
 * The youngest victim was confirmed to be Tim, who was 4 years old.