Warning spoilers from Distant Thunders.
Powder Boys or Powder Monkeys were part of a ships crew during the Age of Sail. They moved bags of gun powder from the magazine to the guns contained in a special bucket with a lid. Powder Monkeys were usually young boys or teens selected for short stature to protect them behind the gunwale of a ship from enemy sharpshooters.
They appear in Distant Thunders during Silva & companions' escape, and Silva must hide when guns fire complicating his plans and allowing Truelove to meet him. Dennis Silva is haunted by the face of one young lad who saw him start the fire just prior to the destruction of HIMS Ajax [DT Pb. p. 369-373].