I received a request awhile ago to do a post on word counts for different age categories in children's lit. Since opinions vary greatly, I did some research to verify the numbers in my head. This is what I came away with. Though, please note that most of the sources I found had slightly different numbers (and I think I'm missing a couple sub-categories). Check out the links I've provided below and read the comments of this post. I'm sure my readers will chime in with their knowledge and opinion!
Board Books: 0 - 100 words.
Early Picture Books: 0 - 500 words.
Picture Books: 50 - 1,000 words. 1k is pushing it.
Nonfiction Picture Books: 500 - 2,000 words.
Early Readers: 200 - 3,500 words, depending on age level.
Chapter Books: 4,000 - 10,000 words.
Hi-Lo Books: 500 - 50,000 words, varies greatly depending on age level. A large number fall between 500 - 20k words. Some 60-90k YA books get classified as Hi-Lo, but I don't think they were specifically written for the category.
Middle Grade: 25,000 - 45,000 words, usually around 35-40k. Longer word counts allowed for fantasy, sci-fi, historical. Up to 60-70k is probably safe (though there are even longer exceptions).
Young Adult: 45,000 - 70,000 words. Longer word counts allowed for fantasy, sci-fi, paranormal, historical. 80-90k is safe (there are some as high as 120k, but I recommend staying below 100k, if possible).
Nonfiction MG/YA: 5,000 - 70,000 words, varies greatly (with some exceeding 100k) depending on the type of book and age level (I recommend researching similar titles to what you're writing/proposing to find appropriate range). Memoirs seem to fall within the same range as novels for their age group.
We all know there are exceptions, but I wouldn't count on being one. I recommend staying within (or close to) the recommended word count for your age category/genre unless you've received a lot of feedback verifying it needs every word (or doesn't need more if you're low). There are a lot of agents that will reject on atypical word count alone.
From my own experience as an intern, I tend to be more critical of YA manuscripts exceeding 80-90k and have to be blown away to want to read a full that length or longer. While some need the length (and those tend to stand out) most simply need more revision and tightening.
Manuscript Length at Kidlit.com
How Long Does a Book Have to Be at Writing for Children and Teens.
Word Count For Novels and Children's Books at the Guide to Literary Agents Blog.
From Picture Books to YA - Information to Get You Started at QueryTracket.net.
Counting Chickens - A Few Words About Word Counts at Hope Vestergaard's site.
On Word Counts and Novel Length at The Swivet.
Word Counts at Stacy Whitman's Grimoire.
Hi/Lo Books: Writing for Reluctant Readers at Writing World (stated count of 400-1200 words).
Hi-Lo Books for Upper Elementary Grades at ALA (example titles ranging from 10-90k words).
Tip! Use Renaissance Learning to research word counts on existing titles. I recommend looking at a large variety to avoid exceptions.