First I would treat them for coccidiosis--you can try probiotics and ACV, but mostly those are preventatives. If those don't work, I would use Corid (labeled for cattle), and throw out eggs for at least 1 week.
Then, if they STILL have "poopy pants" (make sure to clip poopy feathers away from the vent after treatments so you can see if any NEW diarrhea appears), start treating for vent gleet. I have to say, you will come to know the difference between a classic case of "poopy butt" and full-on vent gleet. Vent gleet STINKS, and you will know when that hen comes anywhere near you. There will not only be diarrhea, but an extremely smelly white discharge. The hen may even develop sour crop and have stinky breath and trouble digesting her food.
Be sure to also examine them for external parasites as these can cause the red butt (skin irritation) and diarrhea. Lice are very common; usually you can see the clusters of hard white eggs at the base of feathers near the vent, but not always. You may also want to do a fecal with the vet to determine if it's internal parasites, which have the same symptoms, and treat for those.
Good luck!