The point is that if you can kill Goblin children, you should be able to kill any other children in the game.
It's an argument of consistency.
Both WOTC and Larian are strongly advertising both tabletop 5e and BG3 to all the current SJW diversity stuff for marketing and social media credit, so they should apply those views consistently in this game.
The arguments are not being made because people are being SJWs, they are pointing out the flaws that WOTC and Larian have already made such statements but may not be applying them in this game.
I haven't actually checked if you can kill any of the non goblin children in the game as it isn't something I would normally do in my playthroughs, I've heard from some people you can and this thread that you can't.
However my brain had the exact same curiousity as the OP when I got to Halsim's cell and found out you could kill the Goblin children.