Kids like international penpals, and you can make a little bit of money operating such a service. All you have to do is advertise in a few child and teen-oriented publications around the world, and collect names and addresses of people writing in.
As people write in, you just pick someone on your growing list and send it to them. Obviously, you need quite a bit of volume to make this worthwhile, but it's such easy work, you can charge $5 a person and earn a few thousand dollars for basically mailing a few letters every day. You could also make money by selling your names to a mailing list broker.