The list of Caldecott Medal winners, available from your library, is a good place to start your research. These stories are simple and contain under 1000 words. Many early picture books have been reprinted in the board book format, thus widening the audience. Books average 2-5 sentences per page. Stories are meatier than transition books, though still contain a lot of action.
Chapters often end in the middle of a scene to keep the reader turning the pages.
Kids get hooked on characters at this age, which explains the popularity of series with 20 or more books involving the same cast. Check out some middle grade novels from the list of Newbery Medal winners at your library to get you started. For ages 12 and up, these manuscripts are 130 to about 200 pages long. Plots can be complex with several major characters, though one character should emerge as the focus of the book. Themes should be relevant to the problems and struggles of today's teenagers, regardless of the genre.