in videogames i try to make the same choices as i would in real life. and children typically are rather innocent and unaware of the larger picture of the world around them, punishing them so gravely for their actions would be rather close-minded in my opinion.
there probably wont be a point in a videogame where i feel the need to kill a child and i doubt the game is going to make it mandatory, so no, i wouldn't kill a child.