"Oaths are governed by intent of the speaker, and by the speaker's understanding of what they swore to. You can't make an oath while babbling in another language, but you can make an oath while misunderstanding or not really meaning what you're saying. If you do that, the oath is more literal, stricter, than it otherwise would be. If I said "I swear to disbelieve the words of my enemies" I would not be stuck if an enemy walked up and said "you shouldn't commit suicide right now!", because intent wins. But you said they're saying it as children in a language they don't speak. Intent might not win.
That doesn't mean they're bound to the literal interpretation. It means it's sort of ambiguous. We want them to commit to the literal interpretation, and then it'd be sticky. I expect dropping letters on Angband that say 'we hate Melkor and also the Elves started the war' will result in them revising their oath interpretations. If we shove them towards literalism somehow and then pull that, then we win. By hijacking the enemy's mind-control and mind-controlling people ourselves, of course, but I don't see a better way."