Make a code table where each letter has a code (let the children help you figure it out). Make simple things that can be drawn. Make sure they are clearly distinguishable.
Can look like (not the whole alphabet in my example):
Step 2. Translate the message. So A becomes 
, D becomes 
etc.
Step 3. Write the message on the card and send it with a decoding table so that the message can be deciphered.
The card may look like:
