In the Survival Logbook, we get a deeper dive into Michael Afton's character in which we see he's sarcastic. If we take a look at FNAF 4's Night 3 minigame we see a fat, overweight kid with a red shirt and blue eyes (this detail will be important), who sarcastically remarks to the crying child if he's going to the birthday, “Are you going to the party? Everyone is going to the party. Oh wait, you have to go! It's YOUR birthday! Haha!”, this already connects the 2 characters together. But wait: there's more.

In the Sister Location Custom Night cutscenes, we see Mike walking as he progressively gets more and more rotten. Before this begins, we see that Mike has blue eyes. Just like the Balloon Kid.

It was hidden in plain sight all along.