IME with slapped cheek, only the cheeks are red. Are you sure this is what it is if her chest and arms have a rash too? My sympathies, especially if she has had CP recently too. Look at it this way: she is getting it all over and done with in one go.
I second Cappuccino re not being infectious once rash shows. I was told the same thing when my son had slapped cheek. School insisted he was sent home although he was not complaining of feeling ill, and the doctor told me he was fine to be back at school, so he went back the very next day!