Cold sores on others- touching baby

Julie

Well got to work today and one of the kids has a coldsore on her mouth. (That in typical kid fashion she keeps touching) How can i nicely explain to a 9 year old to not touch my baby because thats extremely harmful to a baby.

I told her its contagious and don't touch, but feel like she's not going to listen as she always wants to play with the baby.