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I couldn't find an exact match, but some of the examples of Badass and Child Duo might fit the bill, as the "badass" half of the duo is probably Rated M for Manly.
Also some of the examples in What Do You Mean, It's Not for Kids?, as a child protagonist implies a child-appropriate story, but this implication doesn't always match reality.
Edited by KD
Seconding What Do You Mean, It's Not for Kids?.
That trope includes To Kill a Mockingbird on the basis that even though it is about children, it isn't very child friendly, which is what you described.
Some other examples are Ender's Game, It, and Blood Meridian, arguably the most fitting example of what you asked ever.
Edit: ok i clearly dont know how to bluetext works pages.
Edited by Nepworks
↑You're supposed to put namespaces before the title (for example, Literature/ if it's a book or Film/ if it's a movie)
Ah thanks.
is there a trope for non-family-friendly stories with a child protagonist?