You know what? I will say, even if they just allowed you to knock the kids out, that would be better than making them invulnerable.
You can do this; that's how they avoid you killing them, if you engage in forced combat with the refugee children in their hideout. You hit them, and they go to 'unconscious' as though you'd used non-lethal, and no amount of extra thwaks will push them past unconscious.
Which, as a side note... shows that Larian are perfectly capable of fixing the bug that causes non-lethal strikes to do lethal damage with additional follow-up damage on the same strike (extra damage die from other sources, etc). They just haven't yet.
