Gentian Violet(sp?) Question

Hi, my little girl is 13 months old, and she is getting a yeasty diaper rash around her vulva. I'm 99% sure it is yeast because I get yeast skin infections sometimes and this is what it looks like. It also looks like the google images I looked up, too. I have looked into her mouth, and I don't see any sign of thrush in her mouth, but she's a toddler and it is hard to see. LOL My questions is, I have a bottle of the purple stuff that I have never used, and can I paint it onto her rash? I am NOT going to put it into her vagina, I just wondered if you could put it onto the skin around her vulva without it hurting her. Also, do you think I should treat her mouth, too, just in case? I don't want it to get worse, and the Nystatin cream I have is NOT working on her. I know this isn't a breastfeeding beyond a year question, but I know that most of you would know about gentian violet(sp?)!

Thanks,
Kayla