There are two different questions being asked here:
1. Should killing children be censored?
2. Does it break the immersion?
Users are arguing with each other based on different assumptions. Let's split up the discussion and answer the questions separately. Note: everything below is my personal opinion.
1. Should killing children be censored?
My rule of thumb is that as little content as possible should be censored, preferably nothing at all. A game shouldn't needlessly censor stuff and I hate it when it does happen. I understand that there are certain laws (especially in some countries) and CDPR doesn't have unlimited (creative) freedom. But even within those laws, a lot should be possible. Besides regulations, the (public) image of the game (and, by extension, the studio) could be in jeopardy when CDPR (dis)allows some (explicit) content. It's never an easy choice.
Regardless, it does set a double standard. We are all fine with playing GTA and running and gunning everyone in the streets. It's not cool behavior, but, nonetheless, we accept it in games. Violence is in our blood, it seems. At the same time, when someone plays, let's say, a pornographic game it's not widely accepted and usually shameful even, especially when it involves -well- illegal activities.
While one could stand on principle to allow the above, the real impact comes from the question below...
2. Does it break the immersion?
That depends on which kind of future the devs want to create. If it's one that shows how little worth a human life is and that death does not make a special exemption for children; if you want that cold hard reality, then yes, for some players, it might break immersion. However, there are other ways to deal with this. For example, having immortal children is always a lazy design decision. Better would be the inability to target children at all.
Either way, there are a lot of things that could break the immersion. For some players, it might be not being able to buy a different apartment. For some others, maybe something as simple as not having the need to eat and sleep. In the end, CDPR cannot cater to everyone and make every aspect immersive. The outcry in this thread mostly seems to be about the first point, while the second one truly matters, in the end.