Oh Gosh, I am so sorry your daughter is going through this. I am no doctor, but have been through enough yeast infections with both my kids that I can help. It always happens to my son when taking antibiotics. We make up what I call our Miracle Cream. It works wonders. Take a small tub and mix up equal parts of:
1) Lotrimin AF (yes it is the athlete's foot stuff). This combats the yeast itself. Generic is fine to use.
2) Petroleum jelly (Vaseline) This acts as a moisture barrier like a regular diaper cream would.
3) Hydrocortisone cream (to combat the discomfort of itchiness and burning).
If you have it on hand add a splash of Maalox, which helps fight acidity in her pee and bowels.
Use this mixture in place of diaper cream at every diaper change for a day, and then three times a day until cured.
You should also cut out any fruit juices in her diet for a few days. Any yogurt with live active cultures is good. YoBaby is perfect. You can also get acidophilus powder from a pharmacy, available without a prescription. You can sprinkle a little of it over/in her foods. I would suggest getting this any time you put her on antibiotics in the future since you know she is susceptable.
And if you can, try letting her go w/o a diaper for a while each day to let the area dry out. Yeast likes dark, damp places.
Please message me if you have any questions. I know how painful this is for them and you watching them. I am happy to help. Best of luck to you.