What are you talking about? You can kill anyone you want in the game. So it's up to you whether you spare the goblin kids and kill the tieflings, or you kill both, or you kill none. Nobody is holding up your hand.
If you can also kill the tiefling kids then theres no issue here, I read you could if you chose to side with the drow / goblins.
So you can kill the goblin kids, the tiefling kids, or none of them correct?