@Captain:
Just to keep this line of discussion going, man Sakazuki did worse than arrest men that were still enlisted and loyal just for not wanting to fight at Marineford.
There is a difference between desertion in the middle of battle and asking to be discharge. Saul deserted, Aokiji left. Plus Akainu is weird. Remember that he shot a boat despite it not being in the government or admiral plan.
Now that we have the chapter out, we can see CP agents openly discussing slave buying and charting out their career expectations for new purchases from a very young age. Seems pretty obvious they're in the business of child soldiers to me.
I am sure they have child soldier if it is the process of enlisting the young impressionable so you can impart your values into them and make them super loyal to them. That's the Doffly system. What I am saying is that they have the same ability to leave that Aokiji or regular recruit had.
I believe they take orphans because they have nothing better and the marines/government give them a better alternative so it is a win-win situation. Hence why I pointed to Rocinante to show how it was not always nefarious(I think in most case they end up in a better place). A lot of the marines seems to come from humble beginnings in fact.
That said, of course not every soldier would have been enlisted and indoctrinated this way. I imagine they're a vast minority, and it's debatable how much the marine characters who are meant to be more morally grounded and likable, like Garp or Smoker, would actually know about this. (I would hope for their integrity as people it's very little, especially for Garp, I see him as a better person than that.)
Garp was fine with the slave shop and was buddy with the Fleet admiral. He took the credits for capturing Roger when it was an hoax. I am surprise how stupid people would prefer Garp to be rather than him being actually that invested in his belief that marines do good work.
Since the CP are a separate branch and seem to be the masterminds it could be that they brainwash or indoctrinate the new acquisitions before enlisting them to basic training to ensure they don't run or anything.
I doubt they do anything more than usual recruit of CP9 or Doffly. It is just that as such age you are pretty much guaranteed someone devoted to your cause. So they are indoctrinated in that way. Brainwashing would be overkill.
Obviously the navy as an organisation doesn't rely on slavery, and Doflamingo was the same, but neither of them are above using it when it suits them. That much is clear.
Doffly owned slave auction.
I am pretty sure marines try to stop slavery non-related to the celestial. The problem with the archipelago is that it is the Celestial background so rampant racism and slavery is more prevalent than ever. As far as using salvery i see marines as doing the same thing as Law with his big guy. Give them an alternative that works for them and the slave. But just like that giant guy could one day decide he had enough and leave Law any recruits could say they prefer the street and go back to them.