My oldest daughter is 6 years old so I started her off with a basic number to alphabet cypher. It was something easy like:
9.19 188.8.131.52 184.108.40.206?
From there I moved her onto a letters cypher. I think on the first one I started her at +2 (A=C, B=D, C=E)
LV WKLS HCVB
In both of these I had the key next to her to figure it out. As she gets better at these I will take away the key and see if she can still figure out what I am sending her in the message.
As for my other daughter who is 4, I was thinking that using a number replacement, or direct letter to letter replacement might be a little to hard for her. So I decided to use pictures as my code, and had her use the first letter of each item to make up the words.
Once she figured out my scribbled drawings she was able to figure out all the scrambled codes that I was sending her. As she gets older I will also move her to using the basic number to alphabet cypher.
I think they next experiment with this will be with invisible ink. Maybe use lemon juice to read it or those