A google search will bring up images of what the rash looks likes. Being a nurse she likely just assume it was a reaction and never thought about a disease since she didn't think he had come in contact with one. I would be calling them to come and get the child immediately and that he can no return to care till the rash is gone regardless of what the cause is. Let them know what another child in care has and that it is important that they get their child checked out because he will need medication if that is what he has.

At this point I wouldnt' let on their child started it. Let them be angry enough to take the child to the doctor so they can come back and say yes their child has it too - THEN you can tell them they started it.