There are lots of ways to do it. I have 9 children with a slightly different spread (15, 12, 11, 9, 8, 6, 5, 3, 1).
In your family I would consider using the books for 7-9 OR the books for 10-12 for your older 3 kids. Or drop the 3rd down with the next group if they would prefer a slightly easier reading level (you know your kids best!). For the rest of the kids I would probably use just the 4-6 books. Then I would pull picture books (nonfiction or fiction) from the shelves at the library each month that fit the time period, and if there weren’t any that month I wouldn’t worry about it for the younger ones, they can listen to the 4-6 books.