Secret Code Maker
Type a message, pick a code, and print a secret note with the key that cracks it. Your message is never printed on the sheet, so there is nothing to peek at. Free, and no sign-up.
0 of 90 characters. Short messages are more fun to crack than long ones.
Pick a code
The sheet
Where does the key go?
Prints on ordinary paper, in black and white. Everything on the sheet is drawn in lines, so it prints the same on any printer.
What prints
Which code for which age
A = 1 and mirror writing work from about six, because both give themselves away: a six year old can count along an alphabet, and anybody who holds a page up to a mirror sees it turn around. The shift code and pigpen suit eight and up, when a child can hold "find it on the top row, read the one underneath" in their head for a whole message. Morse is the hardest to read quickly and the most satisfying to finish, so it is the one for nine and up, or for a grown-up who claims codes are easy.
Every code comes with its key
A code nobody can crack is not a puzzle, it is a locked door. So each sheet carries the key that opens it, written so a child can use it on their own: a numbered alphabet, a shifted alphabet, the pigpen grids, a Morse chart. Under every letter of the message is an empty box to write the answer in, one letter at a time, which is how codes actually get cracked.
Hide the key, and it becomes a hunt
Put the key on its own card and hide it. Now the note is a mystery until somebody finds the key, and finding the key is a game in itself. It makes a good last stop for a scavenger hunt: the final clue leads to the key, and the key cracks a note that says where the prize is.
For a code that is a toy as well as a puzzle, print the free Decoder Wheel: two paper discs pinned together that turn to crack a shift code of any size.
More about how each one works, and the one code to avoid with children, is in Codes and Ciphers That Kids Can Actually Crack.