Something about children ... but what? (spoilers)

Well, I guessed it was not going to be a monster movie, so I wasn't too disappointed over not seeing the "mysterious creature". I understand this film is supposed to be about human relationships and "ghosts of the past". And these ghosts involved some past problems with children, the son of the old couple had died - that much I understood. But what exactly had happened to the son and was their daughter alive or if there even was a daughter - that I didn't get.

This film seems to continue the trend set by Who's Afraid of Virginia Woolf by giving us a view of an older couple who has some sort of sick relationship with their children and who are going grazy about it. Generally that is an interesting concept but it seems that in this film the authors couldn't really come up with any good idea, as to what the trouble with children might be, so they remained very vague about it. (I have not read the book)