@Umesan
That lord is retardo. And it's not like the mages would have to be displayed publicly, they can suffer in a soundproof cart. It's not stopping Captain Retardo from using the children as Mana batteries, it wouldn't stop Lord Retardo from kidnapping and enslaving a rival lord's mages or unsuspecting mages elsewhere.
Plus Lord Retardo has no morals and is a retard, it sounds like the type of thing an arrogant moron would do 1st before wasting time getting outsiders to come as reinforcements and then try to blackmail those Strong outsiders.