Who said they'll be invincible? We just know there will be kids in the game, is all. IS ALL. I hate speculating, so let's leave it at that. And even if so, yes, for me it does the immersion good. I don't like killing NPCs for the sake of it anyways, just expressing MY opinion here though.
How did this work in Fallout 3? Did never attack any of the kids or saw they was attacked, are they knocked down like non killable npc in Oblivion or do they simply don’t take damage?
Obviously they should run away if attacked.