What kind of diet do the cats eat? It is ultimately the best if they are eating a raw food diet, and maybe they are already. Either way, here is a great website for it. http://www.catnutrition.org
Otherwise try adding essential fatty acids to their diet, such as flax oil, Udo's Choice, fish oils or something they will happily eat. Good to offer them a variety. You could try tuna in oil or sardines in oil. Some cats like to lick vitamin E capsule oil, so get a high quality oil.