I don't think a formal health certificate is required, but I would carry their vaccination records with you. When I've had the cats with us in hotels, about 1 in 8 stays I get asked for those.

There is some good info in this forum on getting cats through security, you might want to look for those threads. My felines never leave home without their harnesses on - much easier and safer to grab a panicked cat by the harness than the collar, and you will have to take them out of the carriers for security. Also make sure their tags (they do have ID tags, right? ) have a phone number where you can be reached in transit, like your cell, in case they get lost. If they don't currently have tags, PetCo or PetSmart can make them for about $5.