For readers who have advanced beyond picture books into chapter books, but haven't yet advanced into young adult materials, are often referred to as juvenile or middle readers.
Recommended Online Resources:
Recommended Reading Lists:
Below are some examples of books for various age groups, starting with longer picture books and advancing to middle school materials as well as teaching materials.
Kindergarten to 2nd Grade
3rd to 5th Grade
Middle School Readers
Teaching
Literature Kit Series