I expect illnesses to spike any time a new family joins so I wouldn't talk to them just yet if it was me.

Also, if it was me, I wouldn't have asked her to stay by the door. The amount of time they would be exposing themselves to the others wouldn't really affect them anymore than the child being there all day. If dad is sick and near the kids it's the same for daughter in my mind.

I can see why you did it and even though I wouldn't have, I don't think you were in the wrong either. I would just explain your concern