My daughter had the absolute worst diaper rash that became a yeast infection during a round of antibiotics, it hurt her to even wear a diaper. We tried all the creams and stuff but she had such bad diarrhea that we couldn’t ever get ahead of it.
What actually worked was really simple and fast BUT very high-maintenance and awful: we bought puppy training pads and put them all over the floor and kept her out of her diaper for two full days (except sleeping obviously). This meant that someone had to be right with her cleaning her up every time she went to the bathroom, and we had to jack the heat up. It was honestly terrible I just sat on the floor for an entire weekend next to a peeing and pooping baby. But we noticed a dramatic improvement after the first day and the second day it was basically on its way out. We felt like it was worth the nightmarish 2 days. We also switched to Water Wipes and kept using them after her rash was mostly better. We had some prescription yeast infection stuff we put on when she slept in her diaper. She’s gotten sick a lot from daycare but this was by far the worst thing.