Another thing that works for people is to have some yogurt. The antibiotics might be killing off the "good" bugs along with the bad ones. The yogurt helps to restore the right balance.

Although yogurt is made with milk (which can also cause diarrhea in cats) the cultures in the yogurt should overcome that problem. Most kitties would love to lick up a tablespoon or two of some tasty yogurt - plain might be best. (Don't use the non-fat kind, which has artificial sweetners)

Good luck with all those ailing kitties out there!