There are 2 different types of nappy rashes in broad categories those due to irritation from the urine and diaper, and those rashes that tend to occur in the nappy areas. It takes a Paed dermatologist to differentiate

What you can do as Mums first assume it is a diaper rash from prolonged contact of skin with urine so
1) change diaper frequently when wet not only when there is a poo
2) Apply diaper cream - vaseline, sudocream, shea butter, zinc oxide cream are fine. NOT POWDER pls it must be something that creates barrier between skin and urine powder will be formed into paste by urine and further irritates the skin
3) alllow the diaper area to be exposed to air at times when you are home don't always cover with diaper let there be period off fresh air just blow the area. If you do this, most diaper rash will disappear. ANY RASHES THAT PERSIST ESPECIALLY IF REDDISH, FORMING DISCOLORATION ETC NEED ATTENTION OF A PAED DERMATOLOGIST