PG-13 has a wide enough range these days to be entirely meaningless. There is no harm in taking a 7-10 year old to see an Avengers film, IMO, but of course I wouldn't say the same for the Dark Knight trilogy.
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.
Then there's Spec Ops: The Line. Yikes.
It wasn't until I watched it again when I was 17 or so and I realized how completely dark that actually was.