OP may want to use a bit of discretion before giving this one to an 8 year old, since I think "no graphic violence or mature content" might be overstating it a bit? It's been a while since I read it, but IIRC one of the main characters has "trafficked into sex slavery in a brothel" as part of her backstory, though I don't remember how graphic it is, and off the top of my head I can recall a guy getting his eyeball pulled out by one of the other main characters as part of a torture scene, the aforementioned main character using his brother's diseased corpse to float back to dry land, and a whole bunch of people getting their throats slit/stabbed to death.

The Harry Potter books are plenty violent as well, but since OP said no gore and no adult themes, maybe they'd decide to wait a few years for this one.