A description you can't argue with -- that's still horrible! -- is the way to go here. "Child labor in a mine" is just that. No normal person will defend it.

You lose people when you get six hypotbeticals deep into a philosophical question about the definition of slavery, as you see in that thread. It doesn't matter if you have a point if people write you off when you say it. Step Zero is getting people to consider what you have to say; rhetoric and communication strategies actually matter.