16month son has had pink rash on armpits/crotch for 4 days. Nothing is helping (cortisone/desitin) no fever and doesn't seem to itch.?

Contact Rash. If nowhere else, probably a reaction to something in contact with those parts of the body.Try to see if you can figure out what may be, clothes, detergent, etc.
See your doctor. It is possible this could be a strep rash and need oral antibiotics.