Why did the child back out? If it was because s/he or the parents simply changed their minds, be sure to let the parents (if child is a minor) or the child (if an adult) know of any penalties that would be incurred. They may not be aware that cruises charge cancellation fees, unlike hotels. Knowing that they'd be responsible for the penalty may reverse their decision quickly.
If the child had to cancel for a covered reason (say, an ill relative, or something else covered by insurance), that's a different scenario and I leave it to those more knowledgeable than I to give you advice.