Mail ONE children's book to the first child listed on your letter. (Do it with a manila envelope and using media mail to save money).
Then copy the letter 6 times (move the #2 child up to the first spot, and add your child's name into the #2 slot) and mail them to 6 different people with children. If everyone does what they are supposed to do... your child will get 36 books in return!
Come on... you can afford to send ONE book... are you in??