Is it an evil act to kill Lamm's child soldiers if they attack? I would say no BUT (and this is a big but) I would HOPE any good aligned party would try to defeat them not fatally.
I think it would basically be Neutral, as it is self-defense. However, if the PCs deliberately try to deal only non-lethal damage to the children in order to free them, then I might count that as a Good action (going above and beyond the necessary defense in order to achieve the best result for all involved).