That animals definitely needs a vet and some sort of antibiotic, possibly an antifungal as stated before I've never dealt with this myself, but it doesn't look very comfortable for the little guy. He's probably suffering a great deal. Have you applied the antibiotic ointment? I would do that ASAP and maybe some preparation H around the outside can you get a fecal sample to a vet or lab to see what's going on?
Is that on her side,b/c in picture it does not appear to be on vent but,rather to the right of vent? Separate her b/c some other chick/chicks have been pecking at her bottom(i don't usually like separating chicks,but they are pecking at this)can you put a divider in your brooder so that she can still see chicks,but none can get to her?
Clean gently(skin on chicks is tender,just like a baby)use Hibiclens(available at Walmart or local pharmacy)it is a antiseptic skin wash. Dilute 1 Tablespoon(Hibiclens) in 1 cup of warm water,use a small bottle and squeeze water/hibiclens solution to flush area,clean 2-3 times a day . Then apply antibiotic ointment(nothing ending in cane/caine). If no Hibiclens available,use a diluted peroxide solution,apply the same way.
Is there any smell from wounds/infection? Is chick eating/drinking and pooping normal?
You mentioned other chick had runny poop,make sure you are familiar with Coccidiosis as it is the leading cause of death among chicks.