Have them read Edgar Allan Poe’s The Gold-Bug, which is basically the story of someone going through the process of breaking a simple substitution cypher. I found it enjoyable when I read it about that age. (I actually read a “Children’s version” of the story, but it kept all the crypto, just lowered the reading level.)
Then give them similar problems to the one in the book and have them break it by looking for character frequencies to guess the cypher.
Have them read Edgar Allan Poe’s The Gold-Bug, which is basically the story of someone going through the process of breaking a simple substitution cypher. I found it enjoyable when I read it about that age. (I actually read a “Children’s version” of the story, but it kept all the crypto, just lowered the reading level.)
Then give them similar problems to the one in the book and have them break it by looking for character frequencies to guess the cypher.