If I were Kimball I'd have committed them to the Dam....it's a battle he cannot afford to lose. But then, if I were Kimball I would have cashiered Oliver, replaced him with Hsu, and not needed them (or the Courier) in the first place.
I am not to clear on how Lyons does it. I know he went into The Pitt and killed everyone but for some kids he felt were not mutated and made them BoS, but given their numbers and the amount of young people training. They must kidnap alot of kids or they do let people join but only when young or the BoS under Lyons multiply like Rabbits and if thats the case he need to go back to the west and show them how its done.
They didn't kill everyone in the Pitt.....they wiped out all the gangs and anyone else who tried to put up a fight but there were a number of people left to become Ashur's first followers. Lyons is recruiting Wastelanders, at least since Casdin and his men split off. If you talk to Gunny he complains that they're being committed to combat before he can finish training them to the usual BoS standards so most don't last long. That will probably change as they distribute captured Enclave firepower around the various settlements, begin using Vertibirds to back up thier troops and begin picking off problem groups such as Talon Company one by one. If they take the next logical step....storm Vault 87 and destroy the FEV vats, which means no more mutants....they will quickly become the most powerful part of the BoS....and the most cosmopolitan.