I was about that age when I first read the Fowl books, and they were some of my favorites! There were a few concepts that I didn’t fully understand at the time (e.g., I wasn’t sure what the Russian mafia was when they discussed it in book 2), but overall there’s nothing horribly traumatizing for a child in there. Your kid will be fine!
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, and a nazi-like cult, 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.
I haven't yet either but I've read his Cytonic stuff and while it's not young enough, it's younger. I imagine the Alcatraz one is really well done. My buddies 8 yr old read them and loved them.
I'd pull back the Fowl series for a couple years. It's got complicated issues involving consent, imprisonment, and violence in those situations that might not be very easy to contextually for someone that young. I was advanced for a reader at that age, but I found a lot of enjoyment in the Redwall series. It had great worldbuilding and an interesting setting for a kid just about that age. They might be a little difficult to get them to actually sit down and read though, as they are definitely larger books for children.
It's hard finding kid appropriate content anymore, especially with relevance to wider literature influences. If you dont mind movies, have you considered Howard Lovecraft animated series?