Kerby,
I would most likely put your 5th grader for DITHR in level 4/5 and your 7th grader in level 6/7/8 (as long as they are reading and writing at an average to above average level for their grades).
Just to clarify for the older level, if your 7th grader is pretty independent and comfortable with doing written work, then 6/7/8 is the best placement, however, if that isn't the case, you could also put the 7th grader in Level 4/5.
The book list is divided into genre and also by grade level, so for Biography it gives 10-12 book selections for level 2/3, another 10-12 for Level 4/5, and then 10-12 more for Level 6/7/8. Then, it moves onto a new genre, like Historical Fiction.
You can also check out our book shop on our website for book ideas. If you scroll down on the following link, you'll see books divided into each of the genre's with a reading level listed for each one:
http://www.heartofdakota.com/book-shop.php
Hope that helps.
Blessings,
Carrie