Something like that would be practical.
The Code Book by Simon Singh
The Codebreakers by David Kahn
The Mathematics of Secrets by Joshua Holden
It does assume a fair bit of historical context knowledge, though, much of which probably exceeds 5th grade. Probably 9th graders would have it easier as far as, e.g., familiarity with WWII, etc.
In the same vein, is the "The Adventure of the Dancing Men" [1]
[1] https://en.wikipedia.org/wiki/The_Adventure_of_the_Dancing_M...
ISBN-10 : 1681195143 ISBN-13 : 978-1681195148
Title: Top Secret: A Handbook of Codes, Ciphers and Secret Writing
ISBN-10 : 0763629723 ISBN-13 : 978-0763629724
Mine are out of the house, but I had a few who might have really liked doing something like this.. could make a nice Christmas present, the first mysterious letter..
https://web.archive.org/web/20100128072801/http://www.nsa.go...
(They went offline a few administrations ago, hence the archive link)
You can also jump around the timeline in the archive to see different versions.
cryptoalex•1d ago
P.S. In fact, I was going to send that book to my friend's son who is about same age, and who got some sudden interest in cryptography recently. So it is safe to say that I am now aware of at least two 10 year olds who have interest in Cryptography. I feel a bit surprised, but in a good way :)
mike_richards•1d ago
He started with inventing his own alphabet and language as he was learning to read and write, which eventually evolved into reinventing the Secret Decoder Ring with increasing complexity, so we want to encourage him in his adventure.