I will say that if you can kill goblin kids, you probably should be able to kill the tiefling children. I mean, if you're going to play an evil character, you probably don't feel guilty about killing any of the tieflings and druids. So why would killing tiefling kids bother you? I personally don't like to play the "evil" path. I don't like going around killing anyone in the grove. I have no problems killing everyone in the goblin camp because they're trying to kill me and/or everyone in the grove that I'm trying to save. They're cooking people and torturing them and so on and so forth, so killing them makes me feel like I'm being a hero and purging the region of evil people/monsters. That goes for Andrick and Brynna too, and any other non-goblins in the goblin camp. If the Zhents attack me because I'm killing the goblins, then I'm going to kill them too because they're trying to help the evil goblins.
But killing the tiefling kids, to me, makes me feel crappy, but only a bit more crappy than killing all their adults and the druids and so forth. I mean, I feel like a monster and a jerk if I play that route and kill them all, regardless of children or not. So, I'm not 100% sure why they don't allow you to kill the kids too other than maybe they felt that was just a bit too far and a bit too evil. But then, if that's how the developers feel, should we demand that they feel guilty and make it so people can kill the tiefling kids too? It's their game. If they feel guilty about making it so you can kill tiefling kids - or they think it will cause people to get really upset - then it makes sense that they don't allow it. People can generally come to terms with killing evil monster kids who are killing and torturing and eating people. People might have more issues with killing a bunch of mischievous tiefling kids. So, they may not want players to be able to kill the tiefling children because they may just not want the backlash for allowing such a thing.
Either way, I don't enjoy killing kids, goblin or tiefling, and I don't like killing characters who are good. I have no problems killing characters who are evil: Human, Elf, Dwarf, Goblin, Troll, Ogre, or whatever.
That said, I can reconcile killing goblin kids because I can say to myself, "They are evil and they will grow up to be even more evil." Start making them gray and not evil, and then I have a problem. If you tell me that goblins aren't evil in general and they are just misunderstood and not necessarily bad but just taught to be bad and they could be redeemed and good, I'm gonna have a REAL big problem killing goblins and ESPECIALLY goblin kids.
