If William is busy helping employee's into their costume's, he can't be talking to his son from the private room. There is no other reason for that easter egg to exist, other than to establish that William can't be the one talking or watching his son.

So then, who exactly is it that's talking? The only person who can seem to access the private room without the key is Ennard (and i guess that means any animatronic that is small or flexible enough can get in.) and Mike, and Mike only does so in the fake ending.

I guess he could have a walkie talkie where he is, but that would seem awkward trying to communicate and watch while working.

Anyways, What do you all think of this?