This is an amazing book! It had a great theme and a great plot. The third book is the best (and scariest!) but don't bother with the second one.
Before you let your child read this book, make sure that you know them and their reading ability well. There are cuss words and a lot of violence, death, and suicide. Miranda is 16 and her mom is always fearing that she will get pregnant, though Miranda swears that she would never "do anything". In the third book her older brother gets "married" when he goes to a motel and just meets a girl. He comes home and exclaims that "they're married."
My librarian's son read it and she told me that he said that it was dark and dense for him, and he's 20. Make sure you know you're child's reading ability very well to see if they can handle it. The book describes that as Miranda walks down the deserted streets, she sees piles of dead bodies of teachers, friends, and neighbors. In the third book there is a very gruesome scene where they walk into a house and found a newly dead dog that had eaten his owner alive, and there is a sad scene where a beloved character dies. Though, if you feel comfortable with your child reading Twilight or the Hunger Games, this book should be OK.