As a parent, I read this book for myself, not to the kids. Though I would have no problem letting my nine year old read this book, for some parents who might have concerns about content, let's just start out by saying this is a book about pirates and vampires.
The topic of blood comes up, but the way the good vampires get their blood is civilized and consensual. The bad vampires are the violent ones that typify the violent type of vampire, but there is nothing too graphic in this book. Pirates do engage in drinking and one of the main characters, Conner, who is about 14, does go to a tavern with his shipmates.
As the books progress along, the good vampires are peaceful, but the violence and cruelty of the vampires does increase. But like the Harry Potter books, as the series goes along, the readers also grow and become older and so more mature subjects, like death, grieving and betrayal come up.
It is a well written series with beautiful prose and novel storytelling. Future books in the series encourage applying yourself and the academics of piracy - yes, there is a pirate academy. So many things such as courage, determination, academics, honor, persistence, compassion and intelligence are highlighted in this book series.