I dug through the official movie website, and found the "Production Notes" section. It's a flash site so I couldn't search it for the word "Lyrics", but if you read down the production notes far enough, you get the following text:

Quote:
For the song's lyrics, Elfman went directly to the book. "I wanted to stay as true to Roald Dahl's words as possible. In the book, they were written more like extended chants than songs, but his lyrics already had a wonderful rhythm to them. In the end, I had to do a lot of editing, but I think I was 95% true to the book, with just a bit of tweaking here and there."