Marriage is, by definition, not a private institution, but a public one. Being married doesn't affect what you do inside your own home (to be taken as an idiom for things that don't affect others), it affects how society interacts with you. I therefore think it is disingenuous to claim that it is not a social issue, outside the purview of societal interest.
That's not to say that it should be regulated, in this case or any other, but I merely wish to indicate that the question is not itself injurious, as you seem to say.
You could tell the children that copyright is not absolute, and that its primary purpose is to allow creators to make money off their copyrights. Sometimes, you can use pieces of their material for certain uses. You could follow it up with a few simple examples, like a short piece of a song, a screenshot of a game, etc.
"You tweachewous miscweant!" -- Elmer Fudd