The disease the children have is an actual disease known as Xeroderma Pigmentosum, which is basically an extreme sensitivity to sunlight. It is very rare with roughly a thousand people in the world who have it.
In a pivotal scene, Grace finds a photograph album containing pictures of people she believed to be sleeping. Mrs. Mills informs her they're all deceased, and that people photographed the deceased in the previous (19th) century. In reality, people did photograph their deceased loved ones during the late 19th century. Most were photographed lying down, as if in a deep sleep; others would be propped up in chairs, posed with favorite objects such as children with favorite playthings; adults with books or newspapers. The reason many families did so was because that would be the only photograph they would have of the family member(s) if they didn't, as photography was a rarity in the 19th Century.
Nicole Kidman originally tried to persuade Alejandro Amenábar and the Weinstein brothers to find another actress for the part. Coming off the bright and exuberant Moulin Rouge! (2001), the actress was initially reluctant to do a film that explored such dark places.
To get the kids worked up, Alejandro Amenábar would play scary music when they weren't expecting it.
The movie opens with Nicole Kidman, in voiceover, reading a story. She begins with the words, "Now children, are you sitting comfortably? Then I'll begin." The BBC radio programme "Listen With Mother", broadcast in the UK between 1950 and 1982, always began, "Are you sitting comfortably? Then I'll begin."
Mateo Gil: screenwriter of Alejandro Amenábar's previous film Apri gli occhi (1997), as a dead man's photograph in the album.
Alejandro Amenábar: Appears in one of the photographs of dead people - he's the one on the right with a moustache, of the group of three men.