The Breakout EDU kit now includes the Red Lens Viewer that allows you to disguise a clue underneath a pattern.
The message must be written in blue ink or with a blue colored pencil/crayon. Cyan color is usually the easiest to hide on the paper and reveal with the viewer.
To increase the difficulty of seeing the text easily, you can place the letters, symbols, or other content scattered throughout the page.
Here is a link to the pattern background as a printable file. (Print out the file and write on it with a blue pen/pencil)