First I want to say im not English so sry for bad grammar and all.
Now i will prove why its a prequel:
In the phone call of night 5 the dude mentions that there has a building before this 2 which has called Freddy's Family Dinner which has then shut down (i dont remember why but something evolving the animatronics) after that the GRAND REOPENING has this new building that you play in FnaF 2 and Old Bonnie, Chica, Freddy, and Foxy where probably from there they where then scrapped, deactivated, redesigned and reactived in the new Toy versions, when you finish the 5th night you receive your check which makes the reference to the bite of '87 but in the 6th night the phone guy says that before they close the restaurant they would have another event a birthday party (maybe when the '87 bite happened) and he said this exact word "when the restaurant re-opens I will probably take the Night Shift" so maybe Mike is the Phone Guy and the Phone guy from FnaF 1 is Jeremy and when he dies can make the reference to the bite of '87.
Taking a look at the pay cheque for night 6 in the newspaper its said that they will scrap the new aniamtronics and rebuild the original ones which means that in FnaF 1 the animatronics that are after you are the improved forms of the Toy ones but what really makes the connection is that unlike FnaF 2 that the building is very big and doesnt run out of power in FnaF 1 due to the lack of cash they needed to downsize the building and it runs out of power because most likely they dont have cash to pay for constant electricity and needing.
Now you may be asking about the five kids that died well they died when the restaurant has like in FnaF 2 and in the first game they are mentioning about the events of FnaF 2.
For more proof i would actually advice you to hear to the phone calls and try making the crossovers between both games and Scott now could actually do another prequel where he would explain the events of the Freddy's Family Dinner and what happened there.
Hope you understood my theory and if you have any doubt dont mind asking or disagreeing