I think the "rule" of how much dialogue is the same as word count--just say what you have to say until it's said.
As Alice Pope (Editor,
Children's Writer's & Illustrator's Market) wrote:
For a middle grade novel, the general range is 10,000-30,000 words, or about 100-150 printed computer pages. A book for 10- to 12-year-olds could fall in to the range of young adult as well, depending on the publisher and the subject matter. In that case, a story could range from 25,000-50,000 words, or 175-200 pages.
I suggest visiting a bookstore and flipping through chapters of books to give you an idea of their length and numbers-it can vary greatly. Find books in the same vein as what you're writing.
Not that you asked, but if you're interested in a good article on expository dialogue, you may wish to read
http://www.right-writing.com/child-dialogue.html
Another good article - this one on all aspects of writing a middle-grade novel (although percentage of dialgoue in an mss is not mentioned), is
http://www.right-writing.com/middle-grade.html