Firstly, stop feeding the chick anything but chick starter. Yogurt and other things are going to simply kill it. None of that is helping and probably making it worse. Get rid of the grit and feed them ALL nothing but the chick starter. For the pasted on stuff you are going to have to soak it with warm water. It's not going to come off fast. You need to be patient. It might take a half hour or more to soften it all. Hold the chick and keep the area under a stream of warm water. Rub the surface gently as it softens until it is all gone. Do not put anything on the area even if it looks sore. You are compounding the issue by putting stuff on there that just attracts more debris. Dry the chick after so it does not get chilled and put it back with the others. They should be on clean shavings. Feed them the chick starter and nothing else. No treats .. nothing. I would also get rid of the water additives. The chick starter has everything that they need in it. If the chick has an exceptionally fuzzy butt, then trim some of the fluff away from under the vent. Check them every day to see if the problem is re occurring and wash again if needed. Once they get back on track with a proper diet the problem should resolve.
But when they're raised by a hen, they're out foraging two days after they hatch. Thanks for the advice on cleaning her butt, I haven't had her in warm water for that long.