434 pages is a bit much for kids.
House of Secrets (Fry Boy, buyer review on Amazon): "First, pretty violent for a younger crowd that's less than thirteen, I'd say. Two characters get stabbed, one in the chest, one in the back. Another character gets an arrow in his eye, then pulls out the arrow with his eye attached to it. Another character is known for vivisecting humans."
I'd say this is a bit much for MG, no?
Without intending any offense, this feels like a very 'adult' view of middle grade fiction that middle-grade-aged-me would have found silly.
One of the writers I loved back at that age was Darren Shan, whose books are ALL about grisly deaths. He was a huge bestseller at the time, and still is today, and his books are pretty much all death and gore. [Note: these books are sometimes marketed as lower YA, and it's possible I was reading up a bit, but I see this shelved as MG fairly often and would say it has an MG voice.]
Also? A Series of Unfortunate Events.
Also? The Warriors series about sentient animals dying in horrible and graphic ways.
A lot of MG-aged kids are also 'reading up' into The Hunger Games series, which is definitely YA but is also miles and miles more violent than the book you quoted... and most parents don't bat an eyelid at their kids reading it.
Middle grade readers LOVE violence. It's usually softened in ways to make it more bearable to adults (Darren Shan's characters are usually tearing apart humanoid monsters, always black or green blood and never red; The Warriors series uses animals instead of human characters) but it's still super gruesome. And kids love it!
Violence, I've found, tends to become more disturbing as you get older and add weight, impact, and a sense of loss around it. Violence when you're a kid is just fun and excitement. We joke about lil' kids being unrepentant psychopaths, but truly, when you're young and your brain isn't fully developed yet you don't tend to dig deep into the depth and meaning of deaths in books.