This is something I know a lot about. We have done many road trips with the girls. We have done it going out to West Qld in high 40 degree heat, so it is very do-able to do it in the heat. And there was only one town to stop at, and it was an 11 hour trip.

Portable DVD player is a fantastic idea. Attach the screens to the back of the front seats so the girls can watch their fav dvd's.
Have a box of toys handy to them.
Take pencils and colouring books for them to play with.
We usually stop every 3 or so hours for a stretch. We let Nina go to the toilet, have something to eat and run around. We usually only stop 30 min at the most, then keep going.
Given how long it will take, I would suggest leaving first thing in the morning, say 3 or 4am when it's cooler. Get as far as you can get before one (or all) of the kids starts demanding your attention.