Unfortunately, developing an immunity to coccidiosis is something that all chickens must do. When they are very young they are more susceptible to not being able to handle it. We went through the same - didn't realize that ALL older birds are carriers, we assumed that because ours hd never been sick they could not give the babies coccidiosis. Anyway, to move forward, definitely get some Corid 9.6, I use 2 teaspoons per gallon. Change the entire gallon of water and add new Corid every day for all 5-7 days of treatment. Keep some Corid on hand for future use, coccidiosis is one of those things that always seems to come back. To prevent this from happening to your chicks in the future, feed them medicated while letting them outside for a little while each day. Basically what this does is put them in contact with the parasites, but the medication prevents the parasites from building up and getting out of hand. This lets the chicks gradually acquire an immunity to the disease. Sorry this happened to you!