Everything in the game should be killable. If children are immortal then I do not believe that the devs will resist the temptation to make them troll us from their unassailable invulnerability.
We do not need a repeat of Little Lamplight from Fallout 3.
If they really want to give us ultimate flexibility, then it should be possible to do both "kill everything" and "kill nothing" playthroughs.
(apparently I hit the quote button rather than the edit button)
