Something hard, complicated, and expensive might work: say, a ratcheting minimum age requirement, stipulations that exporters provide documentation on the working conditions at all mines they buy from, extensive investment in the mines, and foreign aid to get early-childhood schools set up with local teachers.

Unfortunately, providing an incomplete or imperfect solution to a problem automatically makes everyone blame you for that problem, even when you provably helped fix it[0]. Anyone who helps fix the problem of child labour by slowly improving working conditions or by brokering a compromise will be blamed for anything and everything remotely connected to it.

[0] https://blog.jaibot.com/the-copenhagen-interpretation-of-eth...