C.K.
My five year old was potty trained at 18 months, but she tends to show occasional redness also. It can be very sensitive. I avoid all bubble bath products. What seems to work is neutralizing the water with baking soda. I bought a huge bag from Sam's Club, and keep it in the linen closet. It really helps. After the bath, I have her put aquaphor lotion on afterwards. It coats but doesn't make her feel like she is putting baby paste on. It goes away quickly. I don't know why it happens, but I think sometimes she doesn't wipe as well as she should and/or she hasn't had as much water to drink lately.
Jennifer