Wasn't there someone claiming that they had to slaughter all the hiding tiefling kids in order to push forward the plot...?
Either way, I don't get why you are so wound up about this. The 'preset' option seems to be going the good aligned playthrough, that's usually what most people do. Goblins have been very humanized in this game, and I have to admit, as much as I disliked them being made so smart and civilized, I found some of them very cute, I grew fond of Zassa or Sazza and I couldn't be mean to her in my evil playthrough, I had to spare her. Still, I'm sorry, but they're effing gobbos and 99% of them are evil (I haven't found a single good gobbo in the game, so I leave that 1% open.)
The children that you can kill, but you don't have to kill (I didn't even manage to catch them in my playthrough, they darted off so fast!) are clearly growing up to be just as evil and already behaving like psychopaths, they are shown torturing other creatures and enjoying it thoroughly. There is clearly a difference between them, and the tieflings, as much as you wouldn't like to admit it, the goblins are still a monstruous race, and their culture revolves around cruelty and back stabbing.