a contact dermatitis would be high on my list. a sensitivity reaction to a detergent (even if you haven't changed detergents) and/or a dryer softener (perfumed bounce for example). even perfumed toilet paper can be a culprit. i suspect her father uses different soaps detergents and may not have a washer that carefully rinses -- guys are not very good at laundry. of course not bathing could be contributory as well.
what would cause a healthy 6 year old girl to have a rash on her genitals that resembles diaper rash? . she was with her father for the weekend and admitted she didn't bathe for 4 days! is the rash a result of not being clean or could it be something else? ! i have not seen this rash on her since she was potty trained at 3.