A good DM won't let you massacre the demon children. A great DM will ensure you don't forget about the moral implications of killing demon children. I remember my party thought the best tactic to stop a fascist invasion would be scorched earth tactics on a critical town. However, they scorched the earth with people in it and ended up massacring the townspeople. I tried to have an arc about trying to atone for that and dealing with the fallout but I wasn't quite good enough to pull it off.