First Stephen King book I ever read was around that age. Through The Eyes of the Dragon. King wanted to write a medieval fantasy novel for his daughter. Personally, I loved it.
That said, if your kid isn’t afraid of a meaty book, It is also a great choice because it really captures how kids can be isolated for traumatic experiences, but can overcome it together. So in my opinion, that made it really relatable when I read it around the same age. It does have that weird child sex orgy in the middle, but reading King is never a good way to avoid reading fucked up shit.
My personal favourite of King though is the Dark Tower series. The Gunslinger is the smallest entry in the main series, so it’s easily readable.
The first and third option aren’t pure horror like It, but IMO that’s what makes them great, because you get little sprinklings of horror, which can make it creepier because it contrasts (without feeling unnatural) with the rest.
Some of my best friends are lactose