I read all of Shakespeare's major plays before I was 10. I read lots of horrifically violent fairy tales before that.

I think you're drastically underestimating the capabilities of children. It's entirely possible to read The Secret Garden and books like it, as a child, and separate the parts that are bad stuff from the past, from the parts that are bad stuff we still grapple with today, from the parts that are good. I didn't have the tools of deconstruction and literary theory when I read them, but I still was able to think about how they portrayed different groups of people, gender roles, etc., and come to my own conclusions, since I was lucky enough to have a large number of different books to read, which helped me develop critical thinking skills, imagination and empathy.

I would even argue that it's necessary to read things like that in order to gain any view of history and people, and the fact that the world changes. Removing every book from the shelves that doesn't line up with modern mores AND that doesn't have problematic content would leave kids with very slim pickings indeed. Also, I don't trust people in power who want to keep reading material away from children.

Ten year olds miming murder? We're meant to be okay with that?

...yes? It's not like Macbeth is a mere game of cops and robbers; it has the message that murder is wrong. It's a very simple play, really. Ten year olds are entirely capable of separating pretend from real and of thinking about things on many complex levels.