I’ve always wondered because I tried looking for on Google, taking some of the lyrics in the book, but was not successful. The closest thing I found was a choral arrangement, but I don’t know who wrote it.
I am not familiar with the book, but there is a well-known children's song/lullaby of that name. "Sweet and low / Wind of the western sea"
Google books: 'Blubber' by Judy Bloom quotes the words '...rest, rest on mother's breast...' which matches words from the second verse quoted here: Wikipedia info on original poem which was set as a part-song in 1863 by Sir Joseph Barnby.