There's nothing inappropriate about the first book; in fact it will read better with a younger audience. Does end on a cliffhanger, so be prepared to feel like you have to read the next book(s). I'm a little concerned about the next two books since they have a "clique" called "The Cutters" -- you do get the sense that people are worried about them, but it goes way too far in the 3rd book.
All the books have a lot of action, but I think much of it is filler. The main story line gets off track in the second and third books; there are a lot of adventures that don't seem necessary to the main plot, which becomes pretty convoluted. Made my head spin.
Really not worth the read; certainly not worth the cost on kindle.