Short version: Can’t kill them.
Long version: You can’t kill kids (which is stupid) and if you did, chances are most players would strip them of everything for their worth in gold, and we can’t have that, now can we? As well, some people said, ‘‘well why not shrink down adult models?’’ Well, there are a few reasons, 1 being that the models would basically be midgets/dwarves, 2 is that, well…children of that time are any male under 16 and any girl under 12. You can see why that would be a problem, can’t you (hint, 12 and 16 would mean they are adults, therefore killable, but modern perspective says anyone under 18/19/21 is a child, so it’s a thin line. Plus, as adults, they can have sex, which would mean there would have to be nude models of them like the adult models (which, with a mod, can be nude. Yeah, it was expected, so Warhorse saved modders the time), and then there’s the whole betrothal/marriage of 30 year olds to 12 year olds. It’s a mess of legal shit Warhorse didn’t need nor want to deal with) So no, children will never be added by Warhorse. It’s unfortunate, but smart on their end.
I just massacred Rattay, killed absolutely everyone except the guard who gives you the tour, the Lord of the place and the trainer guy because they are unkillable…
As for you, they’re immortal because they are essential, yes, as well they are real people so they die (or live) further than 1403, so that’s why.