Before you use anything topical to help with the itching, I'd definitely either call the pediatrician or consult a pharmacist. Depending on what antibiotic the physician prescribed, you may have a contraindication with something topical (meaning they can't be used together) which will lead to more issues instead of helping clear things up......even Over the Counter items.
Unfortunately, Tea Tree Oil hasn't been FDA approved for any medical conditions in the US. I've been researching it for the past year because of its uses in Australia for head lice, and there's no research in the US to show it's beneficial. It's quite expensive, and because it's not monitored by the FDA, you have no way of knowing if it interacts with other prescription medications.
You'll probably be OK with Benadryl, Cortaid, etc. But, please call first to confirm it's OK to use in conjunction with your present treatment.
I was not able to find anything on MayoClinic's website nor HealthyChildren.org to address the rash/itching:
http://www.mayoclinic.com/health/scarlet-fever/DS00917/DS...