Just found this showing the numbers of gun crew members,not sure how it relates to the origin of the 2-6-heave call number 6 being likely to be little more than a child.
See this link to see full article.
The gun was served by a six man crew - known by numbers to make orders easier in the noise of battle. Number 1 was the Gun Captain who aimed and fired the gun. Number 2 used a long spike to turn and raise the barrel; Number 3 loaded the gun and rammed the shot and powder home. Number 4 sponged out the gun, ensuring that no burning powder or waste was left to cause premature ignition of the new charge. Number 5 worked opposite 2 to move the gun whilst Number 6 was the smallest and youngest member of the crew - the powder monkey. Often young boys, perhaps only 10 or 12 years old, the powder monkey collected the gunpowder charges from the magazine deep in the hold of the ship and carried it to the gun.
The whole 3.5 tonnes was now run out, with the crew straining on the carriage ropes to pull the gun muzzle through the gun port in the side of the ship. When the gun came to bear on the target, the gun captain pulled the lanyard to the flint lock. As the flint scraped across the pan a shower of sparks ignited the fine powder - which ignited the main charge and the gun fired, ejecting its iron ball with a forward velocity of some 500 metres per second. The gun would recoil backwards at some 2 metres per second, and the process of cleaning and reloading began again.