As I tell children when I visit schools to talk about creating my books, "A book dummy is not someone who doesn't read books". It is a map of the book. Dummies can happen in stages from very sketchy beginnings with loose thumbnail sketches to more polished versions that you'd actually show someone else (These are often actual size and look more like coloring books.). Beginning dummies are mainly for the illustrator and the illustrator alone. Just like an author may jot down notes in the margins, this is where the illustrator works things out. No need for finished versions here. Get down your ideas, place in lines for text and see how it flows. After a few versions, you can go back and focus on individual spreads and see how things are flowing.
When you are creating a children's book, the standard page count is 32 pages. Here's a loose breakdown...
page 1: (Note... the books begins with the right hand page. It is not a spread) This is usually the title page (often nothing else)
page 2 & 3): (Your first spread) 2 is sometimes the copyright and 3 is often the title with the author's name
page 4 & 5: Sometimes the dedication is on page 4 and the book actually begins on page 5.
page 6 & 7: First spread
page 8 & 9: Second spread
page 10 & 11: Third spread
page 12 & 13: 4th spread
page 14 & 15: 5th spread
page 16 & 17: 6th spread
page 18 & 19: 7th spread
page 20 & 21: 8th spread
page 22 & 23: 9th spread
page 24 & 25: 10th spread
page 26 & 27: 11th spread
page 28 & 29: 12th spread
page 30 & 31: 13th spread
page 32: This is on the left hand side and the book ends here!
The above is a general breakdown of how it works. It is often helpful to check some children's books that you like for ideas.
Board books do not have a standard length. My publisher decided that my new "Hello!" series would have 8 spreads. The above is a very timid initial breakdown of the pages.
Start sketching and have some fun with it. It doesn't have to be perfect the first time out!