I know there are plenty of fellow parents that have to send their kids alone on an airplane, traveling as what is called unaccompanied minor flying. I recently read an article about it, which I would like to recommend to those of you who have to plan for such a thing.
One very good aspect that Jeff Mills highlights in this article is the importance of explaining for the kids what they can expect from the flight. Which is something that all kids flying will benefit from, flying alone or with their parents (or siblings).
You can read more in this article from The Wall Street Journal, called Nanny Services Make flying Solo a Breeze for kids.