You can kill the children of an intelligent creature that is able to communicate, only because of its genes and origin
And you cannot kill tieflings
People would not give a damn about it, in fact, if it was possible to kill everyone, or not to kill any of the children. But we see injustice, inequality. One intelligent race was placed above another intelligent race because of its origin. It's one thing in the gaming world, by Lore bad attitude towards goblins, it's ok. Other when the developers decided it. That children of one race cannot die, and that of another race can.
I would of course kill children throwing stones at a bear with pleasure in a good walkthrough, but how are they worse than tieflings? From the point of view of an outside observer. Maybe I want to kill tiefling children who tried to deceive me and sell the "magic" ring, why can't I do it, but and can kill goblins child?