We made our trip last week. We have DD10 & D3. Our trip is about 7.5 hours. This is how we handled it:
We left home at 3:30 a.m. so that the girls would sleep most of the trip. And they did.
Last Christmas we bought DD10 one of the AC/DC TV/VCR combos for times such as this. I went to Blockbuster the day before we left and got a couple of week-long rentals to take with us. They watched The Country Bears on the last leg of the trip to WDW.
DD10 had lots and lots and lots of school work to do while on the trip. She did an hour's worth in the car on the trip to WDW and another 3 hours on the way back home.
I brought coloring books and crayons for DD3. (She likes to do homework, too.)
Your DD10 & DS8 may like to do the Ad-Lib type games you can get at the bookstore. Do you have any travel size/handheld games they like to play? How about a deck of cards?
Bring snacks - car trips are meant for snacking. Bring the good stuff. Carrots and celery will not suffice at a time like this.
Have a great trip!