The runes carved into the stone over the furnace that the creatures live down spells out "Be Afraid" in Elder Futhark.
Appropriately set in Providence, RI as that was the home of H.P. Lovecraft who wrote the story "The Rats in the Walls" which apparently inspired this film. Though the short story was set in England not Rhode Island.
In the original television movie, Non avere paura del buio (1973), the character of Sally was Alex's wife, not his daughter. In this version, his wife's character is named Kim. In the original version, Sally, Alex's wife, was played by actress Kim Darby. The character was re-written as a de-composite, essentially splitting Sally into two characters. Sally being a child this time was to really reinforce the fairy-tale motifs in the film, and because Guillermo Del Toro felt the story would be scarier if Sally was a little girl instead of a grown woman. Kim's character is a reference to the legendary wicked stepmother trope of classic fairy-tales, though her character isn't actually wicked, Sally simply sees her that way.
The MPAA rated the movie R, despite it being shot with a PG-13 rating in mind. Guillermo del Toro considered this "a badge of honor". When he asked them what needed to be cut to be rated PG-13, he was told "Why ruin a perfectly scary movie?"
Guillermo Del Toro has said that giving the tooth fairy creatures a fairy origin was an intentional homage to the works of writer Arthur Machen, saying in an interview: "I love the Welsh author Arthur Machen and his idea that fairy lore comes from a dark place, that it's derived from little, pre-human creatures who are really, really nasty vermin but are magical in a way, living as they do for hundreds of years. His books are what compelled me to do this." Machen's stories are mentioned by James Mackay's character the Librarian.
Guillermo del Toro: Passenger on the plane behind Sally. He also voiced one of the creatures in the film