My situation was different it wasnt croup but it was a parent that actually had an infectious disease but he told me that the dr said it wasn't that it was something else and so two days later his child comes down with exactly what I told him it was in the first place. Then over the weekend all the dcks (except one) my kids and a set of parents all came down with this disease. When the original parent returned he changed his story and said the dr couldn't confirm it....
So to answer your question .... If you feel a child has something that may be spread to other children you just say to the parent that you feel the child needs to be seen by a dr and you will need a drs note stating that's it's not contagious or if it is what date the child may be readmitted into care.