In the castle of Children's Books, illustrations are king. A children's book author's focus on text and story crafting will likely take far less time than actually articulating their vision for each picture accompanying their story text. I spend a significant amount of time writing detailed requirements for my illustrator. Below is an example of my book cover requirements for Lisa Goes to England and what my illustrator was able to do with them:


I had a very clear vision of what I wanted and put a lot of thought and time writing up enough of a description to get that but to also give the illustrator enough space to let their creativity shine. That way, you'll usually be more than happy with the result.
Here are 5 keys to writing good illustration requirements:
1) Clearly articulate the setting and relevant items/landmarks/features
2) List all of the characters in the scene and what they are wearing (if relevant)
3) Describe any action going on
4) Spotlight on anything in the setting that is featured for the accompanying text on that page
5) Any emotion of the characters that should be portrayed
Armed with a vision and clear and concise requirement writing, your illustrator will get you on your way to your dynamic Children's Book!