J.P.
I was told by my Ped not to use anything for heat rash as her skin is already clogged and especially if they are babies/toddlers. LOs pores are not as efficient as young children, teens adults so you are only going to make it worse - "Heat rash begins with excessive perspiration, usually in a hot, humid environment. The perspiration damages cells on the surface of the skin, forming a barrier and trapping sweat beneath the skin, where it builds up, causing the characteristic bumps."
Here is a good article: http://www.babycenter.com/0_heat-rash_11436.bc
You didn't mention how old she is, but keep that in mind. You need to keep her cool and dry - make sure her clothing is nice and breathable. Give her lukewarm baths to keep that area clean and to help break up the buildup. Let her air dry if you can or at least make sure to dry her off well. Good Luck!