A fun idea for the big kids. This one is pretty involved and takes prep time. (You could delegate the steps of figuring out what supplies you will need to an older child who can tackle that challenge!)
The gist of what you need is…a blacklight/UV light, and a highlighter that you don’t mind destroying. 😉
You squeeze all the ink out of the highlighter into a cup of heated water, then continue diluting it until it no longer appears visibly yellow when painted on paper with a paintbrush, Q-tip, or other implement that you can dip and then write with. But the highlighter will still show up under a UV light!
Like I said — this takes a lot of setup, but is a great deal of fun for the payoff. I bought small UV flashlights on Amazon pretty inexpensively.
What do you think — would your super spy kid enjoy creating their own “invisible ink” and then leaving secret messages?