The option for all of them (both tiefling and goblin children) to be killable should be there. Likewise the option to not kill any of them should also be there.
This is one of those situations where a lot of people have issues with separating the real world from a fantasy world. In this particular fantasy world, being a mini criminal could easily mean death, racism exists, etc. Having a certain subgroup being somehow exempt from all of this makes no sense to me.
Having BG3 identified as "that game where you can massacre innocent children" is something I suspect Larian doesn't want to chance.
In addition the logic of your above quote would equally apply to romance options. We can all agree that would not only be morally reprehensible but illegal in most (all?) markets.
