i dont mind violence against children in video games and here is why.
If you dont want violence against them, dont put them there. Making them invulnerable only break the immersion.
Im a not one usually to go about killing children.. nor anyone in particular (im more like a white knight).
Lets take exemple (since its the only way im able to convey my point by the lack of understanding of certain english words)
1- Baldur Gate: I throw a fireball to kill thugs harassing me in town. I wipe the thugs and everyone around (oops!) but only one child is left standing and hes laughing, having a great day. I still remember it today as it broke my will to play the game at the moment, it threw me off balance. I didnt want the child or any innocent bystanders to die but having the child staying alive like that was the complete opposite, it was frustrating and broke my day.
2- Fallout 3 : Usually when someone need help, either a child (which im usually more sympatethic to) or an advlt, i usually help out. But my first true case of "i want to murder a child" happened in that game, in little lamplight. These are the most irrespectful little piece of lamb i ever encountered. I hate when people lack respect toward me in video game. If a guard in skyrim say "let me guess, someone stole your sweeeeetroll", i just bash his face in. Now you had really really annoying brats and you make them invulnerable? I mean, i got mini nuke, i could destroy the wooden gate to pass. To complete the game, you need to cooperate with them when they are being as*holes? This is the first time i really wanted to cause grief to a child.
The moral of the story, never make something invulnerable, not even a child. If you dont want child murdering, dont put them in the game.