Keep your eye on the Purple House Press site in the coming months; two of the books teased on their current front page are two Alvin Fernald books: Alvin's Secret Code and Alvin Fernald, Mayor for A Day. The first of these two is a GREAT book; it's the first one of the series that I read when I was younger, and I found it really interesting and inspirational. Plus, if they do the reprint right, there will be a huge section in the back containing all sorts of cryptography ideas and exercises, which should be just the thing to interest The Kids in codes and ciphers!
Purple House did a few other Alvin Fernald reprints, which you can see on their Website. One of the books is an entirely new adventure by the late Mr. Hicks, which is unfortunately a retred of Alvin's Secret Code. Still, it was good to have one more story from the man himself, so it's got some historical value at least. But yeah, definitely get a copy of Alvin's Secret Code when it shows up!