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.
There's a hundred different ways this failure of a soldier might get killed, but I would prefer to see the twins reverse the flow of magic and drain him super duper dead instead. (Probably won't happen; innocence and all that.)