Quoting KyliesMommy2012:" DD has a red rash under a roll under her neck. I wash her neck rolls and all that stuff well, I thought. ... [snip!] ... her neck, dried it well, and put some desitin. Any tips or advice on how else to treat i? Ever happen to anyone else's LO?"
As long as it's dry when you put the desitin on, it should easily take care of everything. If it doesn't, it might end up being fungal which you'd have to treat differently anyway. If it's a fungal infection (unlikely), it will get worse instead of better. 99% of the time though, desitin is the way to go. It should clear up in a couple of days.