6 year olds, hell I palyed the shit out those Mario games when I was in my 20's, lol.

Huh, okay...

However, I think an option to toggle the violence wouldn't make sense; I would not let my son play such game even without blood, because the blood is surely not the only reason for the rating.

An option to toggle violence is just a lie.
Games like Postal, Call of Duty don´t belong in the hand of kids or psychic instable persons.

Exactly, that's what I mean.

postal, cod and such games with violence ingame and in cutscenes are different thing

but there's games where you could disable blood, or gore or at least minimize violence.
i had one 2d fighting game. some small dudes fighting. funny game, but with lots of blood. i mean, that was the game kids could play normally but no, they had to put the blood in. option to turn that off would've been useful.

and ofc you wont let your kid play such games till he becomes teenager or something like that.
kids can play sims, sports, and arcade games

That's another true point...
Same for the game to "Watchmen", dunno if someone knows it, was avaiable as download on steam...
Nice game actually, kind of fighting game too, but also with much blood. Would never let a child play this.