You can stop in Palm Desert. It's still too hot for this, but if you go when it's cooler, they have a zoo there. There's also a tram ride or something up to the top of one of the mountains, something like that, I can't think of the name right now.
Also, my kids get antsy so we have a portable DVD player to keep them occupied (mine are a little older), and they watch movies or whatever.
When my oldest was 18 months old, we drove from FL to AZ. We had a ton of toys and just kept changing them out to keep him distracted from the fact that we were driving ten hours a day (with many stops too).
Is the car seat not comfortable, is that why your child doesn't like it or the fact the he/she is strapped in and can't move? Just wondering.
But I think if you can't stop, then try to have stuff to distract the kids from the fact that they're in a car for hours might help. Also having snacks on hand (I use that to keep mu two from fighting. If their hands and mouths are busy eating or drinking, then they're not screaming and hitting (or "bugging": "Mom, he's bugging me.") whatever.
Oh, when I was a kid we drovr down to Phoenix from Northern California a couple of times a year or so to visit family. W eusually left at 2 a.m. after my dad had taken a nap (it's a 10- to 12-hour drive). I did sleep usually since it was the middle of the night.
Also, one more thing, the first time we drove to Anaheim, my little guy had a meltdown when we got to So Cal (we got a little lost in the dark cuz of the construction and made a wrong turn). he love the baby Mozart video and I happened to have the audio CD of it and it calmed him down and he fell asleep. he was just a little tired.
Have fun and good luck!