
As the oldest daughter, willful, outspoken Sophie knew that her life could lead to nothing but mistakes. When her father dies, her stepmother sends her two sisters out to make their fortunes while Sophie continues to work in the family hattery. As she whispers her thoughts to the hats she trims, the shop begins to grow in popularity, until she offends the Wicked Witch of the Waste. The witch casts a spell turning Sophie into an old woman. Sophie leaves the shop and happens upon the castle of the Wizard Howl. He has a reputation for finding young women and eating their hearts but that doesn't stop Sophie from moving in and turning the lives of the whole castle, including Howl's apprentice and his fire demon, upside down in a battle against the wicked witch and for Sophie's future.
I read the first few HP, and thought they were dreadful, and thus never read the later ones, so maybe I'm not the person you want advice from, but here are some recommendations of novels/novelists in the same genre (fantasy novels, written for children, that hold up for adults):
Nearly anything by Dianna Wynne Jones, but I particularly enjoyed The Lives of Christopher Chant, Archer's Goon, and of course Howl's Moving Castle.
Susan Cooper's famous Dark is Rising series. Half the series is more normal-kid (starting with Greenwitch), half is more special-magic-kid (starting with The Dark is Rising).
Garth Nix's Old Kingdom, starting with Sabriel.
While China Mieville is very much not a children's author (really! don't buy a random mieville book for your young niece/nephew, really don't!), Un-Lun-Dun is an amazing book in this genre.
Another recommendation is Howl's Moving Castle (has two follow ups, which are okayish)
you've read Mistborn, are you aware of Cosmere - https://www.reddit.com/r/cosmere/wiki/order
Tamora Pierce has written a number of kid appropriate fantasy books with solid writing and strong female characters. I have seen countless grade school girls get hooked on her work. Some of her books just start to get into more mature relationship themes, but it's enough to embarrass your kid a bit, not scar them.
As a child I enjoyed C.S. Lewis, but was personally affronted when I got to the last Narnia book and discovered that good story had been compromised for christian allegory. Lewis won't make your child a bible thumped any more than Pullman will make them an atheist. Just make sure you know what your kid is reading, and find opportunities to discus the material with them critically!
Ursula K. Le Guin's Earthsea series is another excellent option. I believe when I read it in grade school I found the pacing too slow, so I skipped ahead a lot and inferred a number of plot points. Nonetheless, I'm certain I enjoyed it the first time, it influenced my view of fantasy literature, and I have returned to reread the series multiple times since.
The Hobbit is a great kids book.
Alison Croggon's Pellinor series is less well known, but perfect for pre/early teens that love fantasy.
The Enchanted Forest Chronicles by Patricia Wrede feature a princess who runs away from home to work for a dragon. Solid writing, strong female characters, Pratchett-esque subversion of fantasy tropes. Fun and appropriate for grade school kids.
Howl's Moving Castle by Diana Wynne Jones
That's off the top of my head. Of these books I consider Pullman, Le Guin, and Tolkien to be literature (worth an adult's time), while the others are just fun and well written.
