personally, i love illustrated books, and i think the illustrations really add to the story, if they are well done, particularly for children's books. if you have access to the original illustrations i think by all means you should include them. well-optimised scans shouldn't add too much to the file-size (it will still be counted in mo, not go) and with the cheap storage we can get now it's really not a problem. i vote definitely to keep them.