I have to disagree with the people saying that you can't do power trips or long drives with kids. I do agree that you should balance long driving days with off the road.
My kids are 4 & 7, and we do a long trip every summer. We try to limit our driving days to 5 or 6 hours, but ocassionally do 10-hour runs. The key is to pace yourself on the kids' entertainment. If you start with movies, then by the end of the 2nd hour, they're bored. Instead, let them start the drive entertaining themselves - looking out the windows, talking, listening to music, whatever. As they get bored, add games you can play together - the oldies but goodies such as finding license plates from each state, I-spy, I'm thinking of an animal, whatever. When that gets old, we'll give them a movie to watch on the portable DVD player (you can pick them up cheap at Walmart). Then we'll pull off when necessary for meals and a break.
On really long drives, we'll use Google Maps to find a town about half way with a community park that we can stop at. We'll stop, feed the kids lunch, let them run and play for a while, etc... That does wonders for their ability to continue on for a few more hours. Just get creative, and you can do long drives with the kids without issues.
We've done the Southwest several times; if you're interetsed you can check out our last 2 summer trips for ideas:
http://christineboyce.com/roadtrip2012http://christineboyce.com/roadtrip2011We've done this for the last 4 or 5 summers in a row, so I've got lots of info saved if you have questions.
