Coffeebean, I wish I could give you concrete info but, like calle, I can't help much either. I brought my cat over to the UK in June of 2003. I was so worried about him, but just had to realize that there was nothing I could do about it. I had to bring him with me -- moving without him was not an option. The airlines are good about pets -- when I dropped my cat in his carrier off at the airline, the carriers were all talking to him through his carrier door and being really nice about it. They understood what a nervous mother I was! I had the quarantine people pick him up at Heathrow and take him to the quarantine facility directly, so I didn't see him until he was released a couple of days later. I don't know if the rules are different now, but when we moved, PETS was brand new for the US, so he had to be kept in quarantine so that all his paperwork could be checked out. Anyway, the quarantine facility I used was really great. They kept calling me and assuring me that he was fine, etc.
As for transporting a pet as carry-on, it was my impression was that it was not allowed for international flights. However, I could be wrong. My cat would definitely not qualify as a small pet, so I didn't even look into it. (He weighed 25 lbs at the time.)
All I can tell you is to trust that the cats will be OK. They might not enjoy their travel experience, but they'll be safe and well looked after. And before you know it, you'll be reunited with them and they'll have forgotten all about their ordeal.
Best of luck to you and MrCoffee and the kittens! xxx