Hi, We travel with four Bichon Frises, small white dogs, and the first thought that comes to my mind is use a baby gate near the door so that your cats cannot get out when the door is opened. There has been several of my friends that have had accidents or near misses with their animals when the door was opened. I also crate my dogs when traveling or restrain them with a special seat belt of the sofa. I would think that a crate would be safer for the cats.
Fran