Everyone seems to be treating this as "abrasion:" related. I suggest that you look at a possible fungal souce. Skin yeasts can really go to town in/on skin that has been abraded or chafed. Yeasts and other fungals exist everywhere and love hot damp dark places.
Another source may be the detergent you use--I was told that the surficants, dyes//perfumes in many dtergents cause half the skin problems dermatologists see. My grandson at age 1.5 had a rash that looked like a cross between leprosy and the mange. A change of detergents to a "free" product cleared it up within 1 week. ( It cost quite a few $$$ to get that advice from the Dr.but was worth many times the price!))
Another possibility is fabric softeners in your laundry. They coat the fibers making them less absorbant and prevent them wicking moisture away from the skin.. Many also contain the same dyes/perfumes/etc as above.

All in all a good antifungal and some absorbant powder (Mexana?)(or a combination product) might be a good place to start IMHO.

YMMV- it just works for me/us.