Children's publishing is organised into fairly narrow age categories, each with its own length, structure and reading situation. These are conventions rather than laws, and a manuscript that sits well outside them is a much harder sell — mostly because it won't fit where booksellers and libraries shelve things.
The categories
Board books — roughly ages 0 to 3. Very few words, often under a hundred. Concept rather than plot: colours, animals, routines. Built to be chewed.
Picture books — roughly 3 to 7. The band most people mean when they say children's book. Commonly 500 to 700 words, and rarely above a thousand. Read aloud by an adult to a child.
Early readers — roughly 5 to 7. Around 1,000 to 2,500 words, controlled vocabulary, short sentences. Designed for a child reading alone for the first time, which makes them technically demanding to write.
Chapter books — roughly 6 to 9. Around 5,000 to 12,000 words, short chapters, often illustrated, frequently in series.
Middle grade — roughly 8 to 12. Around 30,000 to 50,000 words. Real plots, real stakes, usually a protagonist at the top of the age range.
Young adult — 12 and up. Around 55,000 to 80,000 words.
Ranges vary between publishers and markets, and the shorter end has been getting shorter for years in picture books especially. Check recent books in your category rather than relying on any single list.
The thirty-two page rule
Nearly all picture books are 32 pages. That isn't aesthetic — books are printed on folded sheets, so page counts come in multiples of eight, and 32 is the commercial standard.
Of those 32, several go to title and copyright pages, leaving roughly 12 to 14 spreads of story.
This matters practically. Your manuscript needs to break naturally into that many beats. If it divides into nine moments or into twenty-two, it will need restructuring regardless of how good the writing is.
Structure in a picture book
Compressed to its bones: a character wants something, tries and fails, tries again, and resolves it themselves — usually with a turn near the end and a final page that lands.
Rising repetition is the engine of the form. Three attempts, each escalating, is so common because it works out loud and because young children find pattern satisfying.
The age of your protagonist
A reliable convention: children read about characters slightly older than themselves. A picture book protagonist is often five or six; a middle-grade protagonist is twelve or thirteen even though the readers are nine and ten.
Writing a protagonist younger than your readers is one of the quieter reasons a manuscript gets declined.
Writing for illustration
In a picture book you are writing half a book. The pictures carry setting, appearance, much of the humour and a good deal of the emotion.
Which means describing what the illustration will show is the main thing to cut. A line saying the dog looked sad is redundant the moment someone draws the dog.
Where the categories matter most
At submission. An agent or editor needs to know immediately which shelf your book is for. A manuscript that is 2,500 words with a four-year-old protagonist and chapters doesn't have a shelf, and that's usually fatal before anyone assesses the prose.