It is much better to start a couple of kids of and let them teach the others. Make sure they really teach netiqette to. Don't assume that kids will do a good job just because it's the Internet and they like computers. They still have to be motivated like when they do other work. Don't promise them keypals within a couple of minutes.
Nobody can garanti that they get an answer.
In our experience there are two ways to chose between when you want a keypal. The most common way is not the one that gets the best result. You can send in an announcement. But often it's the most exciting and unusual messages that gets an answer. You can also answer a message sent to the list.