A lot of it really is this time of year and well, it stinks. My own child had a stomach bug before Christmas, shortly after a cold. Now croup. Doctor said that after the barky cough was gone she would most likely have cold symptoms for another week or two. It's no fun for anyone
My suggestion would be to follow your sickness policy very strictly and send home anytime dcg is in violation of that. Love the 24/48 hour rule that BC was talking about too. For me, I'd require them home the day following the croup diagnosis. They need to rest and here that's not going to happen. They could return after that when the cough was mild and fever gone. Yucky noses etc, they can come as long as its not excessive. Ear infections, not contagious so no worries there unless it affects her behavior or causes a fever. Any fever over 100.4 gets you sent home here, no exceptions. I can also send home for not being able to participate normally.
Anyways, if you want to replace to for it. If they are a great family otherwise, I'd just be very strict on my sickness policy and hope it improves in a few months when cold/flu season winds down!