Oddly, Charlie Heaton gets an "Introducing" credit, even though he had already been in several other movies before this one. The "Introducing" credit is normally reserved for first time film actors.
While Tom (Jacob Tremblay) is in the waiting room, Mary (Naomi Watts) and his case worker are discussing his inability to continue at "Sweetser." Sweetser is, in the words of their website, "... a nonprofit community mental health provider..." in Maine. They're very real, and have been in operation caring for Mainers for decades.
The screenplay for this film was featured in the 2013 Blacklist, a list of the "most liked" unmade scripts of the year.
The ending was reshot. [Variety]