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Hello and thank you for using JustAnswer. May I ask is your daughter in pain from the Atrophic Vulvovaginitis? When asked why she does not want to put underpants on, how does she respond?
It is possible this may be more medical than psychological if the underpants irritate her (and she is irritating it even more). Your daughter may benefit from a vaginal lubricant or anti-itch cream which can help to lessen the irritation and feelings of being uncomfortable, then she may be more apt to putting on underpants and pants. Also for the time being try to see if she would be open to long dresses, baggy sweat pants, etc...overall something loose to at least provide some cover-up. If this is psychological though than a positive/negative reinforcement, and punishment approach would be best. Here is where you want to focus on setting up a tier balance of rewards (e.g. if she can go the whole day without touching or irritating her groin area than she gets an extra treat for dessert), and try to increase the rewards based on what you feel is appropriate. For example if she wears something on her bottom half for a whole day than an extra hour of tv, if she does it multiple days in a row than another good reward like a new toy. You want her to know about these rewards to give her something to look forward to and if she does not comply with it than you take away the rewards as negative reinforcement. For punishment, you want to not yell or smack as that will not help, but instead take away positive things in her life like her favorite toys, tv time, play time, etc...This is not going to be a short process as it could take a few weeks to months to really make a difference in her behavior. A combination of the medical, clothing, and psychological approach could be appropriate too.
Its okay to be frustrated at times (welcome to parenthood), but try to maintain your composure as you do not want your daughter to feel shame as that can only worsen the situation. This could just be a phase or a legitimate medical concern where the underpants cause her to feel uncomfortable, itchy, mild pain, etc...The good news is that this will resolve and you are being proactive in her treatment, so try to be as patient as possible