If you get a chance, read just Lum’s rhyme (from the book) to your child before they’ve seen the book or Lum. Then ask them to draw how they imagine Lum to look. It’s a good exercise in not judging a book by it’s cover, although in this case we’re not judging a character by how they speak.
This is one of the exercises I do during author visits to schools and almost without fail, the children draw giants or monsters. I then go on to read the full story so the children discover that Lum isn’t scary and can change his ways. It is a good exercise for children to understand that not everyone is as they appear.