I took a look at the scene with the goblin children teasing the bear, which is a very IMPORTANT bear, I don't want to spoil it for anyone that may not know. All I will say is DON'T kill the bear. It seems in the cinematic scene we are shown children goblins BUT I don't think that's who you end up fighting. I noticed one of the goblin kids running to get help. So, who you end up killing is maybe one goblin child because the rest are actually adults but since the goblins are the same height, it's hard to tell. I took a look at the faces on the top left of the screen and noticed that they don't seem like faces of younglings but they look more adult.
You can kill the children or leave them to run away and warn the rest of the camp. While killing them is optional, it is still possible unlike the invincible tiefling children.
That's because Tieflings are of Human origin when Goblins are not. They are considered monsters and usually evil. I wouldn't call Tiefling children invincible, look at the cinematic scene with the Tiefling child Arabella, if you fail to save her, she gets killed by Kagha's snake.
I already covered that. If she can get killed by the snake then theres no reason why you shouldn't be allowed to with a weapon or spell. They are invincible when they cannot die and stay alive on 0 HP. Tieflings also look like devils, pretty much the definition of 'monsterous' if you care to look it up. The goblins actually look less like monsters than tieflings do anyway.