I really like this book, even though the topic is homelessness. It certainly seems that the author really looked into the subject --her details are really interesting (for example, the child and his father wear blue because people don't notice blue and the more inconspicuous they are, the better). "Fly Away Home" creates a mood--and if you're interested in letting your children feel a tiny bit of the despair that less fortunate children feel, go for it. After all, you can hug your kids and discuss this book as you go along!
I do feel that some of the other reviewers here are unaware that there are two age categories for children's picture books--4 to 8, and 8 to 12. I would say that this book falls in the latter category. Yes, a very bright six year old could sit through the brief text and come away with the message (homelessness=scary+sad), but they probably don't have the capabilities to really use the information and feelings yet. I would say this is a book to read aloud to say, a fourth grade class, when children really need to start considering social issues and things beyond their little world.
If you're one of these people who only wants books about happy bunnies, this is NOT for you. If you feel that your children can't take the "mixed messages" given by the image of airport security being scary to this homeless child, and you just can't take the time to explain to him/her that law enforcement is not a bad thing, then don't pick this book up. If you don't want your child to feel any compassion for people because you just don't want to make him/her "sad," then for goodness sakes, skip this and every other meaningful book in the bookstore.