The DITHOR guide looks a little different than the other HOD guides, and it also has instruction for 7-15 yo's making it a thicker book. These may be some of the reasons it feels a little different. However, once you get started DITHOR is not hard to implement at all. If you just read the "Instruction" part of the manual first, make sure you have your books chosen, and fill in the reading chart (all things the DITHOR guide walks you through), it's smooth sailing after that.
As far as younger dc beginning DITHOR, with my ds who was quite young when we first began DITHOR, I wrote his answers for him in the Student Book to begin with. After awhile, I wrote half for him, then wrote the other half on a marker board for him to answer. Next, I wrote it on a marker board for him to copy, and last he was able to do it on his own. This progression took about a half a year or more, but made it super easy for him to ease into it. He also gave great answers because he knew I was writing them first

. I think his answers would have been pretty short since his writing was not nearly as developed as his reading at the start.
As far as the Student Book, yes, it is needed as it is how dc respond to their reading at least half of the time, and how they show their comprehension of what was read, the story elements being learned, and the Godly character trait being studied. The DITHOR teacher's guide and Student Book go hand in hand, and without one of them, you only have half a reading program. I've used my DITHOR guide for 7 years now, and it's proven to be an excellent reading program that we never tire of. It is well worth the investment. HTH!
In Christ,
Julie