Croup is caused by certain viruses, like a pp mentioned. As the child grows, their airways grow. Once their airways are big enough, it doesn't get the characteristic barky cough anymore, it is just like a cold. In fact, when dd was 8, we had to call an ambulance for her. Shower, outside into the cool night air, NOTHING worked. The medics gave her oxygen, then albuterol in the squad, then more oxygen. The hospital workers were floored since none of them had ever seen a kid over the age of 7 need to be in the hospital for croup. (Of course, dd has her OWN set of rules, lol.)
If AT ALL possible, do without the steroids. They TRASH the immune system for about 6 weeks.
Anyway, just like any virus, it'll be contagious for several days. I would say for as long as they have wet symptoms just in case. And AVOID babies and small children -- it can make them more seriously ill, espeically if they have any sort of respiratory issues already, like asthma.
Beth