I think it's absolutely fine asking for a doctor's note and if a parent has seen the doctor and it's not contagious than there should be no reason for them to cause a stink over it. If they do, it would be a red flag to me.

We are not doctor's and it's not our job to decide if rashes are contagious or not. We have to think of a group as a whole and keep them all safe.

I don't usually request a dr's note but in the past, I have. Other children were getting sick, their child wasn't getting healthy and they kept saying they had gone to the doctor. When I asked for a note, they all of sudden stayed home for a week!! I knew something was fishy...