Well are you sure its not a yeast infection? If its not (askdrsears.com has pictures to refer to) then I know the best thing is a good long soak in the tub with some baking soda, lots of naked butt time, and squirt some breastmilk on it! (that stuff is called liquid gold for a reason
) I would then let the bumm air dry with the milk on it and then try some Triple Paste if you havent, or maybe A&d? Basically you want to give it lots of time to air out and keep it as dry as possible. I usually coat my little ones bumm once its been aired our for a while and then diaper her. The air is good and helps it dry out. Another thing to try is neosporin, I know it works if Lil'bit gets sores on her bumm. If it persists take your wee one to the dr...he can perscribe something that should help, or give you some other options.
As for the diapers, if its not a yeast infection I would just give them all a good washing. Maybe add some teatree oil or lavendar oil to the wash (both are disenfectants) Or just a good HOT washing with baking soda and vinegar. If you have any sun left by you you can also give them a good sunning, that will kill anything that might be hanging around.
Worse comes to worse, I sometimes give her a day in disposables totally coated in cream. Sometimes they seem to keep her dryier and then I dont have to worry about the cream wreaking havoc on my cds. But then again most times she gets her rashes from being in them soo go figure!