Kelley,
I think you already figured this out, but the books you choose to use for DITHOR are not tied to the student book you choose to use. You can use whichever student book you choose...and then use any books with it!

This past year, I used student book 2/3 with my son in Preparing and he was reading books from the 4/5 boy pack. This worked great for us as adding more writing to the day wouldn't have been a good choice this year, but the books were more challenging and he LOVED them! The previous year we used the level 2 pack and the 2/3 student book when we were just starting with DITHOR and HOD. These books weren't very hard for him to read, but it was a great introduction to using DITHOR for us. He still enjoyed them, but it was a quick, easy part of our day.
I'd think that starting with the 2/3 student book for your 2nd grader would be good. And, I think it would be good to start with the 4/5 student book for your 6th grader. You can then either choose your own books or choose a book pack that HOD has put together. Did you notice that Carrie has a "suggested book list" that comes with the packages containing the teacher guide and student books? That will have some book suggestions for you. Or, we have enjoyed using the packages that HOD sells. I like having the pacing of the books scheduled for me at this point in our homeschooling. And, they're wonderful books that Carrie has chosen!

They've all been a hit here! Here's a link to where you can click on the "optional book packs" of different levels.
http://www.heartofdakota.com/drawn-into.php Now, 2 - doesn't mean 2nd grade. If you click on the level 2 pack, you'll see that the books have a reading level of beginning 3rd grade - 4th grade. If I were to guess what I'd use with your girls, I'd probably start with the level 2 pack and the 5/6 girl pack. And, like Lynn said, your child doesn't always have to be reading a book that is at the very top of their reading level every time. You want to keep them progressing as they learn to read, but I think a mix is good. I personally have loved some of these books that are on Carrie's list!

And, like another mom said, while I can read at a college level, I don't always enjoy that. I enjoy some of these well-written books that are at a 5th grade level, too.
Happy choosing!

Kathleen