Cipher disks can be fun for children and adults alike.
If you spin the wheel to one "secret" position and leave it there, then you can reproduce the same cipher that Julius Caesar purportedly used to protect his military secrets. This cipher is also the same cipher used by "secret decoder ring" toys. It is sometimes known as a shift cipher, or a Caesar cipher. It is very easy to break. Some children might enjoy trying to break it.
On the other hand if the key is securely random, can include any symbol on the disk with equal probability, is as long as the message such that every letter of plain text can be encoded under a different random setting, and if you only use the key once, then you are using a cipher known as the one time pad. The one time pad is I believe the only provably secure cipher so far discovered.
This template might be fun for use with children, since it can work on English messages where capitalization is not important. "SP" stands for space.
This template is useful for working with hexadecimal data. If you find yourself working in hex with paper instead of a computer, then let's hope you have reason for such a high paranoia setting. (Perhaps... cryptocurrency key derivation from public persistent data?)
And here's another template useful for working with passwords:
(If anyone wants SVG source, or python for generating the SVG source, let me know.)
(Updated 12/8/2017 in response to a private email asking for clarification. I still intend to create a better description and analysis, with...
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