Are you planning to do it in one marathon drive or break it up by spending the night in a hotel half-way down? Since the kids are older, you should be able to make the drive without too much trouble. It does get expensive to fly a whole family down! Plus, you'll already have your ground transportation when you get there (no car rental to deal with).
I would let the kids know ahead of time that they are responsible for planning their own road entertainment. They may want to pick up some magazines, tape player and headphones, or hand-held games (e.g., gameboy). Pack some snacks and drinks as well.
Give the car a good inspection before you go. Take care of any preventative maintenance at this time.
Try to make the trip down and back part of the vacation, not something to endure to get to the vacation. If you can afford the time, plan some stops along the way. . .museums, parks, whatever your family would enjoy.
Good luck and have fun.
Peg