Last Saturday I spoke on a panel at the Pacific NW Writer's Association conference about the basics of writing for children. Afterward, a woman came up to me. "I gather from what you said that my 40,000 word novel for ages 7 - 10 may be too long," she said. "Um, yes," I replied. But hers wasn't he longest story I'd heard about that morning. Another woman in the audience had written an 80,000 word manuscript "for ages 8 -12."
Yes, I know how long the Harry Potter books are. But they're the exception rather than the rule. It could also be argued that, as Harry grows older and the books become longer, they actually shift into the Young Adult category.
So what are the word counts most writers for the middle grades should aim for?
The finished page length of the seven books in the two series I've written for ages 7 - 10 ranged from 84 - 133 pages. That translates to a word count of approximately 7,500 - 12,000 words--a far cry from 40,000.
For ages 8 - 12, my books have ranged from a finished page length of 103 - 150 pages, a word count of approximately 11,000 to 18,000 words.
Hope this helps!