> it's almost impossible to effectively regulate externalities in a global economy.
It really is not. Take the EU, for instance. They have the biggest internal market in the world. They can effectively dictate many things unilaterally that companies have to comply with under pain of being locked out of a multi-trillion dollar market (and indeed they do, with things such as health and safety standards, etc.). There's nothing stopping then from properly pricing externalities of pollution for instance, or of mandating that clothes companies pass a workplace standards audit irrespective of where their factories are located. This isn't done because of lack of will.
Even non-global economies end up diffusing the blame for poor labor practices. So many big-box stores end up in a wage-theft situation just because the upper-management sets benchmarks that are only achievable through wage-theft and then they are shocked[1] to find out that the individual store managers are committing wage theft.
Perfection in this, as in all things, may be impossible, but here's a good faith effort to audit supply chains:
All it takes for child labor to enter into the supply chain is one bad regulator and a couple levels of outsourcing. The final "complete" product becomes nearly impossible to audit.