(Spoilers ahead. No names are mentioned, but just the same...)
These books do have a dark side though. The antagonists are quite evil, and there are a lot of sad things that happen to the characters, and it's not the kind of bad think where you can just shrug it off and say, "Oh well, in the next chapter everything will be alright." The tragedies that occur throughout the series have lasting consequences. Some sequences can be a bit intense and may be frightening to children. (SPOILER: for instance, the villain creates his minions by transforming humans into beast-like creatures, causing them to lose their free will.)
Parents should be aware that themes of imprisonment are prominent through the stories. Children are captured by the enemy and snatched away from their parents, and they are locked up in prisons or made to work in factories with no reason to hope for rescue. One character is revealed to have been captured and tortured for quite some time, and he has never been the same since then. Characters are changed forever by the events that transpire, though in the end things are worked out for the best. There are some character deaths too, which may be quite upsetting to young readers or listeners. Parents will be the best judge for their children, since they know what they can handle.
I would recommend this series wholeheartedly to anyone who loves fantasy, comedy, drama, and adventure. Just a word to the wise, though- it is emotional, and so be prepared as you read.
For young readers, I would personally recommend you to be thirteen and up before reading this book on your own. However, as I said, that depends on the parent, the kid, and the family. If you can handle it, great! Because this series is not something that should be missed by anyone.