My theory is that Max and Abby can see them is kinda because the children souls are controlled and related to drawings. Cause when Abby replaced the yellow rabbit drawing to show the children what really happened they turned on Afton. There is also the fact Abby seems to draw her and the missing children a lot of the time so that can also connect to that. And since Max is the babysitter than she most likely saw Abby doing these drawings but thought her friends were imaginary like Mike at the beginning of the movie (I say beginning because when Mike falls asleep at the pizzeria he sees the children in his dreams. But that is my theory.
In "The Man from Room 1280", Andrew attached himself to William, the man who killed him, and messed with hospital staff to prevent them from letting the man die.
Caption this Meme 