I have been thinking of reading them to see if they can be a good read for young kids. Though, I get the feeling (without actually reading them) that they are more suitable for teenagers, then kids of 8-9 years old.
They are indeed for teenagers, but I first read them in my early 40’s and still loved them. The books are full of Greek and Roman history and myths which I love too. I’ve read them twice now :)
I think it's honestly more aimed to older kids, but depending on their reading level and maturity it might be a good read. The books are quite dark, especially in the later ones.