One frigid afternoon in January 2013, a man approached three brothers playing in a field in Kudiya, a hamlet in the northern Indian state of Uttar Pradesh. He had some paying work for them, he said, a loading job, if they were interested.

The Gupta brothers — Amrit Lal, Nakshed, and Akhilesh — recognized the man as Bhola Khan. Khan was in his twenties, and lived with his family in the Muslim quarter of the village. He worked at a local kiln, transporting freshly baked bricks in a horse cart.

A child laborer carries a stack of bricks at…