As far as illustrations it would also depend on if you are self publishing or going with a traditional publisher as well. If you go with self publishing then you the author choose but if you go with the traditional publisher then it's likely they would have someone on staff who would do the illustration work. At least that's my understanding. You will want to make sure your vocabulary isn't too advanced for the age group as well. Dr. Seuss had a vocabulary of some 236 words in his book "The Cat in The Hat". Sometimes less really is more I guess.
Good luck with your project Steven.