This is kind of a non-issue for me. Personally, I don't care, but I do know some folks want to be able to do it. The only reason that I can see putting killable children in the game is for certain role-playing preferrences and for some sort of continuity. This ranks right up there with sex in games. I don't personally care about it and I don't understand the depth of feeling attached to it. But, hey, I hate romances in games which seems to be really popular.
I don't do it, but at the same time the fact that I can do it adds an extra level of realism to the game - to know that all possibilities are, well, possible. By itself this is probably trivial, but lots of decision similar to this (i.e. mature ones) can combine together to really alter the atmosphere of the game.
Anyway, I digress. Or do I?