I use Pepto Bismal. About 20 cc. for every 50 to 60 pounds of body weight, given with a syringe. Once they get a taste of it they usually take it well, as I buy the cherry flavored.
And whenever I have a case of diarrhea, I ALWAYS give them Pedia-lyte as well, as it provides needed electrolytes. I buy the juice kind, apple-flavored, and I squirt a bit in their mouth a few times so they can get used to the taste. Once they get used to it they rather enjoy it, so I put a whole bottle of it in their water bucket. If they have severe diarrhea, I will use a syringe to get a half-bottle into them quickly, but you have to be careful with a syringe with such a watery liquid, so you don't get it down the windpipe or choke them.