Try something anti fungal. Wash all her nappies in lavender oil and use some tea tree on the areas. Dilute the oil first- a few drops in some olive oil will do the trick. Make sure you only use it externally.
Keep the area dry and lots of nappy free time til it clears up.
You can also get an anti fngal ointment from the chemistif you prefer.

But def was her nappies in hot water and tea tree or lavender, 6 drops in the wash and 6 drops in the rinse.