I feel like this depends a lot on the kids themselves, their interests and reading levels, ect. I’ve always been a bit wary of assigning age ranges to children’s literature just because children the same age can have a broad range of experiences and comfort levels, both with reading/vocabulary and with various kinds of subject matter.

There is a story, though, about my sister reading “Alanna: The First Adventure” for the first time. She was six, and there’s a scene in there where the protagonist gets her period for the first time. My sister, confused, asked our mother what was going on, and our mother was rather taken aback. She did her best to explain and resolved to pay a little closer attention to the books we were reading in future.

(I, on the other hand, remember reading right past that scene as a kid. “Women in this book-world bleed once every moon? Definitly just another odd fantasy thing...”)

*ahem*
I read the original Redwall in first grade then devoured most of the series over the next several years, finishing it off sometime in middle school when I finally grabbed the last of the books. So elementary school seems to be a good age bracket provided the reader isn't put off by book length or density. Or violence.

Anything really goes when Games Workshop is writing children's novels set in one of the most violent, depressing, gory, grimdark, non-PG-13 settings ever:

https://warhammeradventures.com

I mean, seriously?

Children?

Warhammer 40,000?

Who the hell thought that was a 'good idea'?

I’ve introduced the eight year old to The Hobbit and he’s been hanging onto every word, though sometimes he asks me a lot of questions that make me wish hS was there. (Can I do that, have a pocket hS to consult for LotR questions?)

The six year old’s class has actually been reading the first Harry Potter book with her class. She was so cute, coming home every day to tell me how certain she was that Snape was trying to kill Harry. Her face when they finished the book was priceless.

I remember I read the last book when it came out, and I was ten at the time. I think that’s plenty old to be exposed to some of the darker stuff;m, though admittedly I didn’t understand how dark it got until I was older.

I’ve been reading the two of them a lot of Roald Dahl as bedtime stories, too. Matilda is the six year old’s favorite, they both love The BFG and Charlie and the Chocolate Factory, and we’re currently reading The Twits.

I’d love to get the six year old into the Princess School series, where Cinderella (“Ella”), Snow White, Rapunzel, and Briar Rose become friends while attending school to learn all the skills a princess needs to know like embroidery and hairstyling, while at the same time becoming damsels out of distress. It was a favorite series of mine when I was little, and even going back to them when I was older, it holds up, even if it’s a bit too simple to properly hold my interest now.

For the eight year old, I’d like to introduce him to the Divide trilogy. It’s about a boy with a heart defect who falls through the titular Divide and ends up in a fantasy world where gryphons and elves and the like are real, bur humans and lions and dogs and the like are fictional. It’s a more obscure series, but another favorite of mine growing up that tackles hard themes about friendships, growing up, and chronic illnesses in a child-friendly way.

(And if I may cheat a bit, their mum and I are planning on introducing them both to Doctor Who soon, since neither of them scare easy and the eight year old especially loves aliens and outer space.)