The advise about bathing is good advise. I know you said sometimes you have your daughter shower trying to prevent infections, but I actually do the opposite with my girls. Even when they are careful, they do not always wipe well enough. If they get busy playing and push it to the last second sometimes they will leak a little bit and then they spend the rest of the day with their underwear damp. This will cause a rash just like a diaper rash in babies when they are left too long in a wet diaper. To avoid this, I put my girls in a bath morning and night.
The other thing I used to do when we lived in a humid climate was leave their diaper or underwear off occasionally to let them air out. If the problem is due to dampness (or humidity) a little slippery elm powder will help to dry out the area and heal it.