I read you could kill children in previous BG games. I remember clearly wiping out all of Beregost just to see if i could do it, and noticing children didn't care about fireballs. Was it changed? players who just want to wipe out the grove to do it 'completely'. It's not worth breaking the rule.
I don't remember any invincible children in BG1 or BG2, are you talking about the originals or enhanced editions? The only invincible npcs I remember were plot-related (those that were necessary for the plot later, like Imoen in the first dungeon) and Arkanis Gath. And they weren't quite immortal, because players came up with a way to kill them and the devs never patched that.