If there's something in this that doesn't make sense sorry because I don't do much fnaf investigating and haven't read any of the books:

My theory is Gregory is the name of the crying child because u here crying in the background of the trailer crying. And then I think he's trapped in freddy Fazbears Megaplex ( ik the name because I saw on the signs in the trailer) but the fredbear plush helps us and he's the person who says 'Gregory be still I think she's found us'.

Idk that's just my theory u have your opinion I have mine.

Like I said if the crying child's name isn't Gregory or something like that sorry I haven't done much investigation in fnaf.

So that's my theory

See ya