Has your vet run a test for Giardia on their stools? That will cause persistent and very liquidy diarrhea and will not go away until the giardia is resolved. It is a protozoa and so a general screen for parasites won't reveal it. Plus, the cysts are shed at various times so it could have been missed.
As for messy cat bottoms: Pampers baby wipes, unscented. They are excellent. You may have to use a few for each cat but it's the easiest way without dunking them.
As for messy cat bottoms: Pampers baby wipes, unscented. They are excellent. You may have to use a few for each cat but it's the easiest way without dunking them.