You can let the gobbo kids live too.
How?
Just... don't kill them? They run away and are gone. If you haven't killed everything upstairs, there are three more goblins coming after that, nothing one can't handle
First, never managed to run them off, they always attacked. But say you can do it (I do believe you, of course, that there is a way and I simply can not find it). If you are saving tieflings, you are killing everyone in the goblin camp. Meaning, those kids are simply killed later offscreen, just like tiefling kids can die off-screen. Unless you are siding with goblins, those goblin kids die.
And the point is, goblin kids do not have plot armour. Tieflinfg's do. Kind of unfair?