For an interesting take on the Hamelin angle, you might try "What Happened in Hamelin" by Gloria Skurzynski. (Four Winds Press, 1979, reprinted a year or two ago in paperback, I think). Nominally juvenile fiction, the author did her homework, although I question hte motivation she gave the piper. And you might want to take a look at Terry Jones' miniseries "The Crusades". The Children's Crusade gets mentioned in the first part. (If your library doesn't have it, it's for sale in the Discovery Store in your local USA-type mall.)
Sorry I can't help on the music.