The employees in Apple's supply chain run from 18 to 32 years old, for the most part. . . if children are found working, Apple's contract requires the employer to PAY for their education through age 25. That is quite an incentive to NOT hire underage workers.
To their corporate policy credit, they do conduct audits. But that has not stopped the practice completely.
http://www.theguardian.com/technology/2013/jan/25/apple-child-labour-supply
That said, Nike has no comment.