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.
The original BG1 i think, computers screen were small and not flat yet :p And there was no entropist. But it's a very old memory, i wouldn't be surprised to be wrong. Although i remember that very clearly.. Maybe i'm going to search for the original game and try it :p
Internet seem to say you can in BG but not in ID. Weird^^