Wow. There are a lot of non-scientific answers already, and I don't know if you've received really sound medical advice. Sorry.
My niece had a nasty diaper rash for months. We were able to isolate and verify that it was candida albicans, the yeast most commonly involved in yeast infections. It went on for a while, she was tested for allergies, etc. When she switched day care providers, it went away. The infection was induced by the stress of being in a class she didn't enjoy.
I'd ask your pediatrician about a compounded diaper ointment that combines an antihistamine, antifungal, anti-inflammatory and antibiotic. Locally it's called Riley Butt Cream and can be compounded at any pharmacy.
As far as tea tree oil - I know there are a lot of moms who are Melaleuca distributors here. I received a personal message from one the other day. I went to NIH's website and researched it from a scientific stand point. Though it's available Over the Counter here in the US, I can not find any FDA approved (different from having a product registered with the FDA) products. There are NO long-term clinical trials on record showing it to be effective in anti-biotic, anti-louse, etc. uses. So, exercise caution and only use after speaking with your physician, pharmacist, etc.
fda.gov
nih.gov