DVD players ROCK. I think you've got your greatest asset right there
I also think you're being very sensible only doing a few hours a day. Alana will get used to the car travel and it'll just become habit.
You can pick up some activities/small toys for the kids before you go (they don't have to be expensive) and wrap them up. Then every now and again produce a package for them to open and enjoy. I did this with Natty for our last holiday about a month ago... A book, a pen, a colouring book, some bubbles, etc.
We went on a 3-week camping trip when Natty was 18mo. I'll preface this by saying I didn't want to go but went because I knew DH was desperate to. So that wasn't a great frame of mind to be in and I didn't enjoy the first few days. However as Natty became accustomed to the camping cot/tent and the new routine (or lack of it, LOL!) it became quite enjoyable. I do have some very happy memories from that trip.
Another positive thing is that the kids get to spend heaps of time with their dad. Not sure what your daily routine is like and whether your DH already has lots of time to spend with them, but I have found this to be the most fabulous thing about our holidays. DH has all this time to bond with the kids and take part in all aspects of their daily care. It gives me a break and gives them all a perfect opportunity to enjoy one another.
I think the boys will have an absolute ball! I would happily do another trip like that with Natty now, but Phoebe is my weakest link atm, LOL! You will have an awesome time.
How fabulous that your grandfather will be 100! What a wonderful celebration you will all have.
Take loads of photos!