From Celebrating Children's Books, Arnold Lobel adds the following in his chapter, "A Good Picture Book Should ...". A good children's book has a simple, artfully, skillfully composed narrative that will hold up under numerous readings [I know one picture book that ended up under a refrigerator, hidden by a disgusted reader from a younger sibling who couldn't get enough of it]. It should include strong characterization, humor, poetlry, drama. It should have drawings that aren't too adult, nor too cutesy. That artwork needs to match the subject and mood the text establishes, and, as I stated, the art needs to integrate well with the story. Finally, the children's picture book, like all good fiction, must be true to the lives and passions of the author and illustrator.
For more on this subject, I recommend the aforementioned book. Also, the best way to learn is to grab a handful of children's books in a style you admire and review them critically and see what makes them work. When Michael and the New Baby was first written, it was a black and white book. The publisher asked for at least half of it to be in color. I researched color long and hard before starting this project. Dr. Seuss's books were tremendous guides, showing how a limited palette could accomplish much when properly used.
Good luck to you, author, and to your illustrator