Bah, just wrote my reply then it dissapeared!, try again.
I have 2 cats, and I travelled them from London to Houston, a 10 hour flight. This was on top of a 4 hour ride to the airport, an overnight stay in a kennel, and a couple of hours paperwork the other side. They were both old cats, they were at least 10 when they did this. THEY WERE FINE!! honestly I didn't notice any stress with them at all, when I picked them up they were just lying in their boxes, of course I couldn't let them out to wee in the airport, so make sure you take a litter box with you, so you can let them out somewhere safe and enclosed (even if its the back of your car) to have a wee - and if not, I used a large tupperware style box, and put litter in it, and put it in the box so they could wee.
They did go in the cargo bay, I forgot to mention that, but it was december, so it was cold, rather than being too hot, so like the others said, check whether that will be ok, maybe you can clip a fan to their cage or something, because they don't let you put ANY food or water in with them. I reduced mine's water for a couple of hours before the trip to make sure they wouldn't have a full bladder, and if they are outdoor cats, make sure you catch them a day before LOL
They do freak out - be prepared, for the first few minutes they are in their crate, but they settle down pretty quick.