It has been conjectured that Der Kindestod may have been inspired by Freddy Krueger, the antagonist of A Nightmare on Elm Street (1984). However, Joss Whedon says that this character was "based on a 'this thing creeped me out as a kid' thing," as with many of the monsters on the show. In general, the monsters were suggested by the vague bogeyman figures that bedevil many children.
The book on the cover of which Cordelia finds the image of Der Kindestod also contains the passage on an unnamed demon belonging to the Brotherhood of Seven that Buffy and Sid defeated in Buffy the Vampire Slayer: The Puppet Show (1997).
The trivia item below may give away important plot points.
Xander's unresolved romantic feelings for Buffy, which began in Season 1, are brought up, first by Angeleus, when he attempts to enter Buffy's hospital room; Angeleus taunts Xander about his love for Buffy and how it must irk him that Angel took Buffy's virginity. An annoyed Cordelia later tells Xander that his loyalty to Buffy is not attractive (his later love interest, Anya, will make similar complaints) and how she knows that he took friendship with Buffy as way of being close to her, since she didn't see him in a romantic way. This may explain Xander's anger and frustration over Buffy's romantic choices throughout the series, as ordinary guys like him don't seem to "do it" for her.