My daughter is 8 months old and has had this rash on her it for while now. We put butt paste and some destin on her and it seems to fade a little but will come right back. It's not gotten to the point where it's going to her legs and up her back ): I wash her clothes with dreft, but I am not sure if her father does. They are just red little dots all over her butt, not pimple like or anything, just plain dots.
I am dealing with something similar with my daughter. First I would say buy her triple paste. That worked better than any cream I've used. Then you could check into buying jock itch cream which is just a fungus killer- it kills jock itch, yeast, and ring worm, all fungus infections. You can ask ur dr about these things and see what they say. You could also just check into triple paste AF (anti fungal)