I know how you feel but I think cats adjust pretty quickly. As long as you trust the clinic where they'll be staying, try not to worry. I've had to board a cat for several weeks and then fly her across country and she was OK, though she did get rather fat at the kennel! I also have a friend who moved to Europe from the US and her cats had to stay in quarantine for a loooong time, and they were fine. I think it's great that you are making the effort to keep your kitties with you, a lot of people wouldn't bother.
"I love cats because I enjoy my home and, little by little, they become its visible soul."--Jean Cocteau