if it's a yeast rash, you should try diluted grapefruit seed extract. when DS had a horrid yeast rash it only took a few applications to knock it almost completely out, and within a couple of days it was entirely gone. i'd be extremely leary of putting that cream on a baby's genitals since OTC yeast infection cream makes my crotch burn, itch and swell - can't imagine what it would do to a babe who was sensitive to it.
you can also use calendula cream after you've applied the GSE and let it dry, it doesn't create a moisture barrier and is antimicrobial which also helps with the yeast. plain yogurt can also be used, and is very soothing.
lots of naked time will help clear up contact rashes, if it's not yeast, as will changing as soon as baby's wet.
and you might want to give baby oral probiotics - often times yeast is not only in the diaper area but also in the belly and can also show up as thrush. better to make sure it's not systemic, so it does not return or get worse.