As a kid I was super into cyphers and codes. I've invented several, alongside my friend Meagan. We got some good stuff. Getting into song titles and stuff, too. But before I got there, I did a twist on this activity.
The pigpen cypher is a simple and adaptable substitution cypher, because it can be adjusted by moving around the four maps.
Here's a nice simple intro to the concept on counton.org. To introduce it to kids, explain the first grid with letters and get them to fill in the next three. Give them a written out note to decode (in this case, I used "Hello Detective, You Win!"
Once they've got it mastered, invite kids to write other notes in code, or to come up with other codes.