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.
Monsterous races are fine to me and most players, but WOTC have stated that none of their races are meant to be inherently evil anymore. If they want to push such agendas, then they should enforce them as well and at the least remove the ability to be able to kill the two Goblin children. And likewise I actually began to somewhat like the goblins in this game, particularly the one cheering for Volo, and even most of the evil ones actually end up seeming somewhat funny, but then the game lets you kill their children.
Its ok to let the stealing Tiefling kid die to a snake, then what about if Lae'zel wants to smash the skull of the one that tricks her with fake magical rings? Or if Astarion wants to mercy kill them rather than leaving them to die on the road after being kicked out of the grove? Or if you happen to role playing a character that simply wants to kill anythkng that looks like a demon?
And on that note how are tieflings not a monsterous race as well when they straight up look like demons with actual horns? Who decides what the moral and ethical scope is meant to be in a D&D setting?
'Tieflings are invaders from hell that need to be destroyed like the devilkin they are and sent back to hell' - what is wrong with roleplaying such a perspective? Do the Githyanki spare children when deciding to slit people navel to throat as Lae'zel so exquisitely details?