Artemis fowl is great at that age.

There’s still the occasional image, like in the series, the fairy language is read in a spiral, so there’s an image of it with a simple cipher so they can translate it themselves, and other bits like that.

The characters are funny, yet the plot remains serious.

While someone did point out there’s some serious things in there, they’re not actually explicit, it’s not worse than any other book. Harry Potter has murder, slavery, violence, Ave nazi-like cults, but that doesn’t mean it’s an adult-only book. It’s about the presentation of these themes. Artemis Fowl IS fairy Die Hard, but it’s not rated R, it’s written for kids, and I highly recommend it for yours, it was perfect for me at that age.