I think people are underestimating Vin Diesel's current star power. If you're going to pay for Vin Diesel it's for a major role. Someone whose been the lead of so many major blockbusters doesn't come cheap. Which is why I think Vision and some of the other characters are a little far fetched, unless he's just lending some sort of voice talent.
I think maybe you're underestimating 12 year olds, as people often do. There's no 12 year old who hasn't seen a bullet to the head in some media or another. You can say that's not okay, and I might even agree with you, however there's no ignoring the fact that this is relatively standard for entertainment. And I feel like trying to say that 12 year olds need protecting from this is symptomatic of a misunderstanding of 12 year olds. Sort of like how PG-13 movies can't have boobs in them, because that would obviously be traumatizing to a 13 year old boy.