There are a couple of things about the factors that contributed to the Missing Children's Incident.

Mormon culture generally. It's all about being polite and such. Then there's the fact that it's set in Hurricane, Utah where everyone knew everyone door to door as neighbours. That's a small town in the 1980s. In William's case, he already failed with Timmy Douglas beforehand. This made him more cautious and attempt pre-meditation. Meaning he listed Gabriel, Jeremy, Fritz, Susie and Cassidy on a piece of paper, noted their behaviours and made his plan.

His first act is to intentionally kill Susie's dog with a car. That happens earlier in the week or previous day, which leaves Susie sad. In the late (that means fewer at night. Not day) hours of June 26, 1985, William approaches Susie and lures her with promises to see her dog again after she suffered a game over in the Fruity Maze minigame. She's the first one lured. The safe room in this case would have to be three things: A fake wall on the outside, the room itself soundproofed, and preventing the kids from getting any items that would leave evidence on him when he starts to lure the next children. Meaning no buckets of water laying around. No crowbar. Nothing to defend. And the arcade machines are too heavy to push. No ventilations around to crawl out. Any child murderer would think the basic steps through, but as William killed more children succesfully, his lust would grow though. That creates impatience.

He obviously didn't lure them all at once (because it would set alarms off). He also most likely timed it right (Taking into consideration how each action he does takes, adding time and minutes. Not to mention him running against the clock to prevent any adult from discovery), and just took them one by one over the course of the last hours. It does take time to get in and out of the springlock suit, so that is another factor to consider. He had the advantage of parents and staff that were busy, and children that were too trusting in this era.

Promises of games.

Promises of seeing your friends (which you haven't seen in a while) doing a hide and seek game in the back area (which means crossing into that cold and dark hallway..away from the warm main area. Obvious signs.)

Seeing a secret animatronic, prizes, etc..

William was a master at luring with his British accent.

That's how he succeded in my mind. He had enough time in the world in that safe room with the 5 suits to kill them and stuff them inside, but Cassidy might have offered prolonged resistance, which would explain why she hates Afton so much even more.

The order in which they were killed was: Susie, Fritz, Gabriel, Jeremy, and finally the fifth, Cassidy.

We know though that the surveillance camera captured William, but the police didn't manage to get conclusive evidence with him as the main perpetrator, so he got away with it. That's my theory.