How we did it:
- white crayon
- watercolor paints
I gave my daughter a "blank" piece of paper and told her there was a hidden, secret message on it. (I had written the message beforehand with a white crayon).
I told her that, to reveal the hidden message, she would have to use the special secret agent paints! (They were really just regular watercolors).
As she painted more and more, more of the message revealed itself. She started to point out some of the letters!
She was delighted when the hidden message was revealed. From one secret agent to another. :)