I think the children being mortal is an acceptable request and a somewhat different one from seeking the ability to deliberately kill them. On the grounds that it is in fact a game full of pixelated beings, I don't find the latter necessarily offensive, either. I think the case can be made for immersion in such a way. Having the kids be vulnerable to bandit raids, dragon attacks, vampires and other hostile creatures isn't a question of morality, in my view. It's about making the game that much more realistic. It's a bit illogical to have all of these crazy things happening, adults dying in their villages, and meanwhile the kids can run though dragon fire with barely even a reaction. I think them actually being susceptible to injury and even death is a device an RPer could put to good use, in good, or less than benevolent ways.
All of that said, given how comparatively rare child deaths are even in films to this day, I don't see game designers implementing it with any real frequency or on any broad scale. Hell, even GTA doesn't have kids running around in the world at all (NPCs are by and large all killable). I can understand wanting the ability to do what you want in your game, however. I just don't see it gaining too much traction in the future of game development, because child death has nearly always been seen as a sort of line in the sand.
