There is, of course, the possibility that the game designers might just feel guilty about killing tiefling kids, but goblins, who are traditionally evil, they have no problems with.

Maybe, for the sake of the consciences of those making the game, we can even just accept it for what it is.

I'm putting myself in the shoes of Larian employees. I, as a DM, would have a REAL hard time letting my players be butchers of kids who are not evil. However, man-eating kids who are evil, yeah, I'd have a bit of a problem with, but I'd allow it. There's a big difference between killing the mischievous thieving tiefling kids and the man-eating goblin kids. A BIG difference. Larian is DM. If they don't feel right about it, then they don't feel right about it.