Your whole post begs one question: why would you want to kill children?

because it can be fun
who did not nuke that MF child in fallout 3? laternight or something
or placed a timebomb for childeren to steal in fallout 2.

thing is it's fun it's artifical

Role-playing evil characters without it being weird that the average in-game 17.5 year old is 10 billion times more invincible than both 18 year old you and the main plot's MegaBoss you spent 100hrs levelling up to defeat? I mean there are mods for Skyrim that allow that so it's not that weird / unusual a request. All they really do is remove the "Essential NPC" flags from everyone (as they used to be in Morrowind before the hand-holding). It's certainly far less weird than Fallout censorship of literally removing every kid from the game. Or designing an Open World Role-Playing game where "you can be anyone you want to be as long as it involves role-playing only Lawful-Good characters..."

OP is specifically asking for the possibility to kill children.

He also refers to games in his youth "where you could kill children"...as if that was something to be proud of, but doesn't mention that even in his youth, this particular "feature" was highly unusual and very rare.

He then ends his rant with the statement that the only reason you're not allowed to kill children, is because Steam (and probably everyone else not allowing kids to get killed) wants to appear as "virtue signalling".

I can't answer for him nor understand his intentions, but a lot of people mod games to be more open / less conditional not due to some death fetish, but simply because they find RPG's in general absurd to play when your bullet-proof, stab-proof, explosion-proof, fire-proof, frost-proof, fall-damage-proof, poison-proof, radiation-proof, magic-proof 17.5 year old completely invincible next door neighbour is 10 billion times more suitable for saving the world than your 18 year old fragile "Chosen One" self. Why not just send him on the main quest instead whilst you do his menial job? LOL...

Open world RPG's are regularly advertised as being the most "immersive" type of games yet regularly turn out to have the least believable / most immersion shattering / plot hole inducing environments in existence due to stuff like this. People often seek out Essential NPC Removal mods for that Morrowind like experience of "Be careful who you kill as there may be unforeseen consequences to your actions" or "if you truly want to be evil, you can be" rather than the dumbed down hand-holding of modern RPG's whose "evil-ness" comprises mostly "you picked the rude / sarcastic conversation response and that's that".

Are you asking for rationality in fantasy games?
Well, then: here's the rational explanation for your question:

Because if the (according to you) "bullet-proof, stab-proof, explosion-proof, fire-proof, frost-proof, fall-damage-proof, radiation-proof, magic-proof 17.5 year old completely invincible next door neighbour" would save the world - there would be no game for you to play.

Btw: why this strange fixation on a 17.5 year olds?
I'm pretty sure the OP wants to kill everyone - regardless of their age.
Pregnant women, babies, toddlers, kindergarden children, teens...I mean, why make distinctions, if all you wanna do is kill, right?

I find that worrying.
Especially considering the rise in school shootings, where "18 year old, mentally fragile, self declared "Chosen Ones"" (to re-use some of your fixated terms), choose schools with young children in particular, because they make such easy and soft targets.

Sorry guys was drunk damn X_X

As you said yourself, it's a fantasy game. If you feel that strongly about death in video games, you could always just stick to Tetris. Absolutely no-one to kill at all there. I mean, even Doom can be "taken" the wrong way for "encouraging" you to kill sick & disabled adults, right?...

Ah yes - the age old excuse of "it's only pixels!!1! Nobody dies for real!11!!!1!"
Tell you what - that's the answer I was expecting since the moment I typed the word "fantasy" in my comment.

Well they don't. Nor do people "die for real" in thousands of violent action / horror movies. Seriously though, if you're genuinely that sensitive to violence in video games, then simply don't play any game with violence / weapons in. Problem solved. Complaining "GTA is violent" though is like complaining 'water is wet'...

One thing to end this here: it's not violence in games per se, that I have a problem with.

But, I already noticed - you're very intend on missing the point at hand.

So, I'll simply leave it at that. There's no point in reiterating what has already been said.

In a videogame? Why not? Why do you want to do anything you want to do? Show us a list of all of the games you play so we can pick it apart and judge you.

Feel free to judge someone all you want but don't complain if someone does it to you. Are you willing to be judged?

I think it's mostly because of the ratings. Killing sub-18 year old characters instantly means an 18+ rating, which most devs/publishers will try to avoid if possible. It sucks, but as ratings affect the sales and higher ones generally sell less, that's why you get immortal children and such. In the past, people didn't give much of a damn about it because gaming wasn't mainstream. Now that it is, mainstream rules also apply to it, sadly.

Ahhh, that's an easy one. If the game gives you the option, it's something you want to try. Easy as that.

Law of the playground: If you buy a set of toys, you want to play with all of them. You leave out one, it's money wasted.

If you can kill children in game, you will. So I'm actually okay with not being given the option.