The answer is: it's up to you. I always pre-read books that may have questionable content, and cannibalism definitely raises a few red flags in my mind! The book was well written, not too wordy for young readers & maintained a good pace with suspense, but definitely not any "horror" type content (like Goosebumps). I was impressed with the author's way of presenting even unfavorable characters (such as the leader of the cannibal tribe) in a caring and respectful light. A great reminder that even people who look to us like "bad news" are the lost lambs that the Good Shepherd searches out with great love and compassion.
Although the word "cannibals" is used often to refer to a tribe of people, the only real mentioning of what cannibalism is occurs on pages 16: "'Cannibals?' Beth asked. Her face was pale. 'The ones who eat people?' 'I don't think there's any other kind,' Patrick said." and two other times, but only briefly, stating that the practice was "part of their religion" and that they would "kill and eat" their enemy. No details, so use your discretion based on how sensitive your children are.
The book did a good job of helping the reader get into the mindset of a different place, culture and time period, and at one part explained quite well (in a way young readers would understand) that missionaries were willing to risk their lives to share the Gospel in dangerous areas because they have already "died" and now live in Christ, hence death is not a consequence to fear because to die is gain.
When I feel my kids are at the appropriate age, I will definitely let them read these books, as I feel they do an important job- tell our kids the stories of missionary heroes in light of their Christian faith. I think the books would spark great conversations about sharing our faith, and spark interest in learning history & geography as well.
Also, at the back of the book, readers can answer several questions about the text to get a "secret word" which can be entered online, claiming, "you'll receive a prize." Unfortunately, I went to the site to enter the secret word for this book & for some reason if you click to enter the secret word for this book ("Battle for Cannibal Islands" ), the screen that comes up prompts you to enter the secret word for a different book in the series, "Attack at the Arena". I tried entering the secret word, in case the link was correct but the text hadn't been changed, but it came back as the secret word being incorrect. I'm sure they will catch this error and correct it soon, but in the meantime, it will be pretty disappointing to kids who are looking to receive a prize :( I wrote an email to them letting them know the problem, so hopefully it's resolved quickly!
In the interest of full disclosure, I received a free copy of this book from Tyndale Blog Network for the purpose of reviewing.