Explanation from original OP:

Context: in the 1600s there was “hearth tax” placed by the king on homes with multiple chimneys. This lead to builders constructing small and labyrinthine chimney flue that all connected to a single chimney in the building to avoid the tax.

The result meant master sweeps would hire (or kidnap) young children to clean the insides of these tiny, winding chimneys. They would often get stuck and die from suffocation. The occurrence happened so often that people would say a crying sweep boy is a good sign because it meant they were still alive. A silent sweep boy could mean he was already dead.