MILK is VERY harmful to cats.
Whether or not you think that your cats love the stuff, you should never give them any dairy...if you want to treat them give them catnip or cat grass, at least those two supplements are actually good for their digestion.

We write off diarhea much too easily in our automatic pill-popping society...diarrhea is a VERY VERY serious sign of illness in humans and in all animals...If you are having regular or chronic diarrhea it is the sign of a serious disease...diarrhea is your colon trying to flush all the toxins out.

So the fact that your cats have diarrhea is much worse than just you "having to deal with the stink of it."
it means that your cats could potentially get very very sick...so why keep giving it to them? even in small amounts?

we tend to anthropomorphise our companion animals, but they do not "want" treats and vices as we do. If anything your cats would much prefer running after a toy mouse on a string then to drink milk...or even better they would much better catch a real mouse...but that is beside the point. Cats are not glutons they are carnivores and hunters, so if you want to make them happy play with them, don't give them something that you have convinced yourself that they want and that makes them sick.

remember for you it's just the smell and texture of dhiarrhea, but they are the one who have to actually suffer through the diarrhea.
I love my cats and would never want to make them sick, especially not deliberately.