That is one long trip! I would suggest that you spread over two or three days travel. I am from MI, and we went to AZ every summer, it is sooooo long. Leaving early is OK, but they will be exhasted once you get to the hotel and it still might be very late once you get there to get to the half way point. We did this trip from Texas to AZ and took two full days.
If you also have some of the wonder color pads and pens (no mess!) An Ipod for the 10 and 8 yo might be good when the movie selection is below their age level. Books for them too, and snacks. Car bingo cards are great-I found some at a cracker barrel gift shop once, but you might be able to find them on line before you go, or make some yourself. Looking for cars, signs, landmarks, etc. It is fun. Also, the abc game for the older ones, and a good deck of cards with a spare if they loose a card are good.
M.