We're in the planning stages of doing exactly that (June 2011). We made the drive in 2004 when I was pregnant, this time we'll be showing a 6 year old the west!
We had family in El Paso at the time, so we stayed there a day each way. It's a logical choice, but there is something about crossing the state line that first day to feel progress has been made! We're thinking Deming NM that first night. On the west side of EP, right off the highway is Leo's if you want some great Mexican (NOT Tex-Mex!) food for dinner.
In 2004 we went down to Bisbee and stayed the night in the Shady Dell before going on to San Diego. We'll most likely just drive straight there this time, spend a few days there, and head up to LA area.
We headed straight back last time, LA to Palm Springs to Phoenix, EP again and home. On the way back this time we're thinking of checking out the Grand Canyon, Petrified forest and Carlsbad Caverns. A lot depends on your schedule and travelers. I have a little cave nut, I think Carlsbad blows Inner Space Caverns away! In Holbrook AZ, near the petrified forest is a Wigwam motel that looks JUST like the Cozy Cone in Cars, only with teepees instead of road cones. Looks like a fun place to stay, if not a little cozy.
Is anyone planning to go to Legoland? I hear mixed reviews and I'm thinking maybe just the zoo (which I've been to before), and maybe Sea World (never been to SD or SA seaworld). The southern Cali Citypass has admission to all, seems like a better value.
If you've never made the drive before, please make sure you have plenty of water and the car is ready for emergencies. It's amazing how desolate it is in stretches. We had a flat tire a few miles out of Tucson, if it had been a little further down the road we would have been out of luck, no cell reception, nada.
I'll be checking back during the planning to see what kind of ideas y'all come up with!