Doesn't Endgame literally have Thanos being decapitated? I don't think it'll scar most kids but I don't remember any decapitations happening in the Dark Knight movies.
A quick, clean decapitation of a bad guy is hardly going to set a permanent mark. Nobody cared when Scar also died at the end of said movie.
It wasn't until I watched it again when I was 17 or so and I realized how completely dark that actually was.
Then there's Spec Ops: The Line. Yikes.