Oski, I think that having them with you is an invaluable thing to do. And also, THANK YOU for taking them with you, as so many people these days abandon their pets just when they move from one apartment to another, never mind across the ocean...
I flew with my dog from Israel (13 hour flight), he was my carry-on, and what I did is give him a pill to make him sleepy before loading on to the plane. I think you may have to stop feeding the cats alittle while before going on the plane, not sure. You better ask your vet about that, I know it helped me alot, because he didn't get sick and since I didn't give him water before the flight, he was ok and didn't pee. I also snuck him out of the cage while everyone was sleeping and kept him on my lap, under a blanket...Not sure if your kitties will stay put. LOL
I tried sending you a private message but was unable to, I guess because you are very new to the forum...so I will post here what I wanted to say to you:
Oski, how big are your kitties? I have a carrier for small animals that is somewhat different, in the sense that it is lower to the ground but wider that the usual ones. It could be a nice solution for you. Contact me if you are interested, my email is listed in my profile.