As Nancy, Glen and Tina pull up outside the school early in the movie, there are a group of young girls nearby playing skip-rope and singing a song, the first lyrics of which are "One, two, Freddy's coming for you." However, at this point in the film, Freddy hasn't killed anyone (in the dream world), and is unknown to the main characters. So how are the children already singing about him?
The key seems to be in the dialogue between Tina and Nancy as they get out of the car. Tina tells Nancy, "It was so scary, and when I woke up it seemed like he was still in the room with me", to which Nancy responds, "It sounds like a real boogieman, one, two, Freddy's coming for you", to which Tina says "That's what it reminded me of, that old jump rope song."
The fact that Tina refers to the song as old suggests it has been around for a while, and as such, the song is most likely derivative of an urban legend about the real life crimes of Fred Krueger. However, over the years, the name has ceased to have any real meaning (none of the parents talk about him), and the children thus sing the song without any knowledge or understanding of what they are singing; Freddy Krueger is thought of as a prototypical boogieman, not a real life killer. Obviously, Nancy and Tina have no idea of the details of the real Freddy or they would have recognized him from the dream, which lends credence to the suggestion that the real man has developed into an archetypical representative of the boogieman in the cultural zeitgeist of the area; everyone knows the song, but no one knows the real details.
This has certain parallels in reality, where real killers such as Lizzie Borden, Jack the Ripper, Charles Manson or the Zodiac Killer have become almost metaphorical expressions rather than descriptions of people who were once really alive. Compare this with names such as, say, Harold Shipman, Fred West or Gerald Stano, names which tend to evoke images of flesh-and-blood people. Freddy has obviously come to reside in the first group. Hence the song is about him, but no one really knows that much about him.
There is one problem with this theory however. After the opening line, the song goes on "Three, four, better lock your door / Five, six, grab your crucifix / Seven, eight, gotta stay up late / Nine, ten, never sleep again." If the children are singing about the real-life Freddy, these lyrics seem to make little sense, as at this point in the film, nobody is aware of Freddy's dream abilities. As such, there are three theories to explain this. One is simply that the lyrics do make sense; in that "Gotta stay up late" and "Never sleep again" could refer to how Freddy abducted children from their bedrooms, and how, in the middle of the murder spree, children were afraid to go to bed incase Freddy got them. A second theory is that the children singing the song aren't supposed to be interpreted as literally existing in the same ontological reality as the characters. Proponents of this theory point to the fact that when the camera is on the children, things are in slow motion, and the colors distorted, with white in particular bleeding into the surrounding colors; as such, the children are simply a symbol, they aren't literally there, and the song they are singing is something which has been taken from the old jump song to which Tina refers, and changed to reflect what is going to come. In this sense then, the children would serve as a chorus of sorts. The third theory is simply that the filmmakers goofed by having the children singing anachronistically about events which haven't yet transpired.