enterococcus diaper rash-desperate for help!

My son and daughter have had a diaper rash/yeast infection for over three months. The most recent culture spotted enterococcus-don't know which spp. They're finishing up their 8th day on septra (my son is allergic to penicillan drugs) but it is not working. They are so miserable. Pustules, itching, the whole bit. Now I'm reading that, if left untreated for too long, this bacterial infection could lead to meningitis, endocarditis, etc. I'm terrified and my dermatologist is out of ideas. Please help me. I only have two more days on this antibiotic and I'm afraid when it's over the infection will get worse-if that's possible.