Was her DD coughing when you went over there? If she wasn't coughing then it would be more difficult for her to transmit the disease.

Again, if your son wasn't coughing, then again it is unlikely if he did have it that he would have passed it on.

If it was at all possible, i'd stay low for a couple of days at home. Have you looked at what the incubation period is?
If he has to go to care, then i think it would be fair to let the carer know about the situation.