Children's books are generally 32 pages long. That includes 1-6 pages of end pages and front matter such as copyright info, dedications, and one or two title pages, so in the end, there are usually only 26 pages for actual story.
When I get the text for a book, the first thing I do is mark the page breaks and jot down some image ideas. Then I pull out the storyboard template I created in InDesign and adjust the box dimensions to fit that of the finished book. Here's a basic pdf version you can use:
I also like to figure out where the text is going to go on each page, so I will cut and paste it right into my thumbnails like this:
It's too tiny to read at this point, but when I do my larger comps, all I have to do is scale the image up and the text is already in place. I mark off the end pages and front matter, so this usually puts us starting the story on page 6. At this stage of drawing, it is best to work very small and focus on composition rather than detail. I also like to decide at this point where I will have double page spreads, full bleeds, borders, or vignettes. I try to keep a variety of angles going on to keep things interesting. In the end, I have a nice idea of how the book will flow.
Here's an example of the storyboards I did for the book, "Sensing Peace."
Some of the images underwent major changes before it was published, but in general we stuck to this plan.