Its a video game thing and because its taboo. Some games have allowed the killing of children or have depicted a scene, and people in general don't take kindly to that sort of thing. While we can sit here and rationalize it, the majority of the population doesn't agree and it would be bad for business (in most cases) to have that much controversy over a beloved franchise. Maybe in the future when people don't see it as taboo it will be something they will let in, but this is the first time children have been part of these games and its best to take baby steps for now.
No kidding.
She may have been turned sometime way back in the 3rd Era, but she
looks like a child. That's where people get the hang up.
And going off of this gentleman's post, every game in the past that has allowed child killing/sacrifice has either been quickly pulled from the shelves, or had its name blasted across the media so hard that the company that made it never fully recovered.
I agree that she
is not a child, but since she
looks like one, you will never be able to kill her. That's not just a Bethesda thing, that's something 99.9% of the Video Game Industry (indeed most industries, why do you think 90% of the time kids don't die in horror movies?) holds as a standard.
It would probably be best if you just got used to it. In no modern game are you going to be able to ice/grease/kill/maim/whack a child. It'll never happen.