Step Four - Illustrations
When you decide on an illustrator, you'll need to tell them what size you want your book to be.
This is a link to the possible sizes you can print using KDP. When you pull out a ruler and a few sample kids books, you'll see that you're actually pretty limited in your choices.
In regards to spine size and text (a lot of illustrators ask this) your book has to be over 100 pages to have anything printed on the spine, which ins't a usual size for children's paperback books.