That requirement was for the multiple contractors whom they found had unintentionally rather than deliberately hired underage workers, and was a condition of retaining Apple business. This was in addition to required training in how to avoid hiring underage workers, and establishing procedures to avoid this.

Looks like that applies to _new_ violations, with new contractors and new contracts I assume.

A contract clause withholding outstanding payments until certain conditions are met would do the trick.

I suppose such a contract would work provided the cost of fulfilling those obligations did not exceed the outstanding payment. Otherwise I would expect the company to just cut their loses.

It may not be perfect (which country's is?), but China has a justice system. A court case with Apple's weight (and money) behind it probably would be reasonably succesful in China.

Yeah, local government enforcement is the most likely way I can see this being effectively enforced.