One mistake a lot of cat owners make is only _ever_ taking their cats along in the car when the cat is going to the vet. _That's_ why cats s often react as they do, puking, peeing, and all that wonderful stuff. Of course that's not the only reason, but it's a conditioned response, very often, of a cat thinking it's going to the vet. One method of preventing this is simply to take kitty along on other trips so that he/she doesn't learn to associate car rides with the veterinarian.

In your case it sounds like getting the cat a sedative is in order. That seems to be the most common advice given, anyway: sedate the guy.

It's great that you're planning this far in advance. I just googled "pet travel airline" and got hits for pet transport services and information on specific airlines policies on traveling with pets. Hopefully any pet service you consider would be willing to provide a list of recent clients you could contact for feedback. Your vet or local vet association may have recommendations. A local zoo may give you some good advice, since they often ship small animals. Since you have time, I'd work on desensitizing your cats to travel and not count on drugs. Tranquilizers etc can reduce your pets ability to respond to travel conditions, such as regulating body temperature. I know you prefer in cabin air travel, but if you do opt to ship the kitties, be sure to research weather limitations with the carrier. In August the concern will be to assure that temperatures are not too hot at both ends of the route. I always hope for the best, but expect the worst. In that vein, it might be a good idea to know in advance the phone number and location of the 24hr emergency animal hospital closest to your destination airport. Once armed with that information, you'll never need it. I'm sure with advance planning, your pet move will be as uneventful as possible.

Perhaps most of this is impertinent, but I felt inclined to share.

Indeed cats *can* go 6 hours without a litterbox, but I'm not sure it's the healthiest thing. How do I know, you ask?
I feed my cats in their crates (the way you would a crate trained dog). The eat all of their meals there. The happily leap into their crates and not so patiently wait to be fed. These guys have learned to LOVE their crates. Well, I have one girl who has been obese, and we had to get her as close to her ideal weight as possible. So, she got closed into her crate and stayed there until everyone else was done eating. Once in a while, my fiancee and I would both forget that we had not yet let her out before leaving the house. We would return in the evening and say, "Where's Friday? Did you let her out this morning?" We then race to the bedroom, let her out, she would saunter out to the litter box, then return and jump right back in her crate and start asking about dinner.
My siamese mix had a pure phobia of crates when she came into my life. She would starve before she would stick her head in to reach food. So, we borrowed a large dog crate. She used that, while the other two used the cat-sized crates, placed side by side on top. She slowly adjusted, and finally entered her large crate as happily as the other two. Eventually, we made her comfortable jumping up into the smaller crates as well. Now, when we go to the vet, she still views it as an ordeal, but the crate has become a safe place away from home. She still cries and yowls, but no longer pukes, poops, and pees in the crate. She finds car rides stressful, so I keep them to a minimum, letting her find comfort in her crate.
My adventurer, Torque, started his life with us in arrangement, and was a tender 4 months old. We put him in a harness, on a leash, and started taking him out at 6 months old. He has no idea that cats don't do these things. He rides in the car well. He accompanies me to the farm, and plays with the barn cats in the hay loft. He has outgrown any "cat-sized" harness, so now walks in a collar, just like a dog. He loves to go to the vet, or anywhere else for that matter. His brave and outgoing attitude makes me want to look into a therapy cat program. I think he would be excellent.
My point is, people expect dogs to behave, but assume cats are untrainable. I have learned differently. You can't train them the same way, or to do all of the same things, but you an work with their instincts and your loving encouragement. You can do your best to communicate your expectations, and they seem happy to try for you.

Since you are planning to move in August, you still have lots of time that you can use to ease the journey by making their crates a safe haven. Who knows, perhaps your kids will show an adventurous tendency like Torque's when given a chance... :-)
Best of luck!