Would you let the other one go to a party?

DS, 4yo, seems to have HFM or something equally bothersome with sores in the mouth. Our family was invited to a neighbors house for a water play party with a bunch of girls from DD 6yo class.

When I thought DS just had a run of the mill cold I was going to take DD but when I figured out DS was seriously miserable I decided to keep DD home as well in case she's got his germs. At this point she has no signs of illness but the two kids snuggle and share stuff (despite my warnings) quite often.

Is my self imposed quarantine too severe?