It is always good to create a barrier between the childs skin and urine on the diaper. desitin works well and vaseline is another barrier. Leaving the child with out a diaper so the air will dry it up is also helpful.. ..
This will sound odd, but Vagisil is made for yeast, and it sure can work like a charm on a baby's bottom if he or she has a yeast-based diaper rash. I always used to have it in our diaper bag, which invariably made my girlfriends laugh.
Also, have the baby bathe in cooler water with no irritants like a bubble bath or even soap in the area a couple of times a day if possible. Prior to the bathe, use mineral or olive oil on cotton to gently remove any existing lotions or creams you may have used (leaving them on can trap bacteria in there). If possible, let baby "air dry" with no diaper on, outside if warm and sunny. Don't use baby wipes, use a spray bottle and pat dry to clean during diaper changes.
My son has had terrible diapper rash and antibiotics diarrhea, here is what we do:
Clean the skin with liquid Zantac, it's a good antiacid, let dry a minute or two
Apply butt paste - we use TriplE Paste
Use natural cotton insert between butt and regular diapper
One other suggestion from some of the books I read when Augie was a baby was along the lines of the Zantac. It is to take regular Maalox and let it stand (so it settles), and then to take a cotton ball and dip it in only the liquid at the top of the bottle, and to dab that liquid on. Something about the anti-acidic nature of the antacid. Seemed to work pretty well the few times I tried it. The Boston Children's Hospital worked up a recipe that was half Aquaphor (the skin cream) and half Maalox (shaken this time), beaten up with a mixer. They call it Aqualox, and it makes a great bottom paste.