You might try doing some searches for folk songs regarding child labor, factory workers, early unionization efforts, etc.

DD sometimes obsesses with ideas that most would consider dark for a 6yo, so I can completely relate. I dread driving past the local 9/11 memorial, since it gets her started with the incessant questions about what happened, how, why, and so on. My too-sensitive self doesn't want to think about it, thank you.