On the few occaisions my DDs have had bad nappy rash I've used an antifungal cream (like canesten) along with sigmacort which is a weak steroidal cream you can get over the counter, which soothes rashes etc. Usually clears up over a day or two using these.

If you don't get a response from using these, please see your dr as it is possible to get a staph-like infection and it may need ABs to clear it up.