It stated out back in the 1980's when Freddy Fazbear's Pizzeria was known as Fredbear's family diner. There were only two animatronics, Fredbear, and Golden Bunny (who was later known as Springtrap).
It was your usual everyday kids play place restaurant, similar to 'Chuck E. Cheeses', but one thing made it better than all other family diners; their newest animatronic allowed a restaurant worker to move some parts aside and lick them in place, allowing them to climb inside and walk around in it, entertaining young children.
Fredbear would take the stage doing his usual routine, singing, dancing, all that stuff that you would expect. Then, a worker would walk into a backroom and put on the Golden Bunny suit and walk around talking to the kids and entertaining them while their parents watched and ate.

The owner, Freddy Fazbear had four sons. The older three would often tease and bully the youngest child, Jonathan. Their father hardly noticed because of his restaurant.
Then, that day came.

It was late at night, and the four children were playing in the empty diner while workers cleaned up. As usual, the older three bullied and teased Jonathan in the back room. As a joke, the second oldest son, Miles picked up Jonathan and pretend to put him headfirst into Fredbear's mouth. But he tripped on Golden Bunny's hand and accidentally dropped his brother's head into Fredbear's open mouth. The weight of Jonathan's head pushed a spring which make Fredbear's mouth slowly close shut, crushing the young boy's head. What the other two boys saw was Miles throwing Jonathan's head into Fredbear's mouth. The three frightened boys ran to their fathers office and told them what happened. Fred was tired and thought they were joking, but when he saw their faces, he knew they weren't lying. When he's saw what had happened to his son, he immediately asked, "Who is responsible for this!"
"Miles threw Jonathan into the bear's mouth!"
Freddy immediately believed them and struck Miles across the face with his hand.
"Why would you do this?!?!" He interrogated.
"I-i-it was-" Miles chocked through tears.
"SPEAK UP!!!" Roared their father.
"I w-was j-j-j-joking." Stuttered Miles.
"Joking? JOKING!?!? YOU CALL THIS A JOKE?!?!" Barked Fred. "DO YOU HAVE ANY IDEA WHAT HAPPENED?!?!"
Later that week Fredbear's Family Diner was shut down.