Try Aveeno Oatmeal baths! If it is heat rash it will help with any itching. The bad things with rashes if the doctor or nurse knows it is not poison oak or ivy they will almost always say it is contact dermatitis. At least that is what they always tell me.
But let her soak in the oatmeal bath it will sooth the skin thats irritated and also I use one of the packets and make a paste and just put it on the most irritated parts, I find it helps with the itching and when she is done soaking just shower it off
Hope this helps!