The fact that you can't kill tiefling kids isnt because they are good, their alignment, or because of anything to do with storytelling. it is an oversight of political correctness issues like in many other games where they make children unkillable for political and legal matters, such as some countries banning games where you can kill children.
But the presence of the two killable goblin children show that they have completely overlooked this, and are applying that logic inconsistently. I do not think that any legal issue would arise it all the children in this game happened to be killable, just as they already were in BG1&2.
'Evil' in D&D doesn't designate what you get to kill or dont get to kill, as you are meant to have the choice to play as good or evil yourself. The fact that we can join the goblins and create a raid on the Druid camp shows that we can clearly play as evil in this game, and as such there is no reason from a lore perspective as to why only the evil goblin children should be killable.
It doesn't affect you if another player chooses to do an evil playthrough and kill everyone, children included in the Druid camp, just as much as it doesn't if anyone does the typical same thing with killing everyone and the children in the goblin camp.