Milk products are helpful at this point because they cause a mucous buildup that helps coat the intestines to provide relief.
When your chickens go through this and then recover they will be resistant to coccidiosis, but keep in mind that coccidiosis has something like nine strains. They will be resistant to what is in your soil, which could be only one or any combination.
Corid or liquid Amprolium treats all nine forms of the cocci protozoa. Sulmet only treats a few, and it is known to be a little harder on the chick's body than amprolium, so if you can get, in my opinion, Corid is the way to go.
If they are not eating, you might also try egg yolks or mixing the yogurt with their feed to get them to eat it. Most of all, make sure they drink that Corid water. That is what can give them relief.