Precocious kids always run into this problem, OP. They're either exposed to more mature themes, or have to settle for basics. The original Winnie-the-Pooh stories by A A Milne may be the exception - the adult themes are well hidden, and on the surface they are full of innocent humor and whimsy.
At that age, my kids started reading Harry Potter and The Hobbit. My parents never forbade any books, and neither did I. I read aloud The Chronicles of Narnia, The Hounds of the Morrigan, lots of fantasy and adventure. They read or were read many Roald Dahls.
If you're the one reading, please also be mindful of having a storytelling voice. You need to slow down, articulate, do all the different voices, some sound effects, be extra dramatic. It's a real skill. Don't be afraid of modifying the text or adding an explanation if you know a concept will be unfamiliar. My kids could listen to practically anything when I switched on my storytelling voice