Magic Potion Trick For Kids I am currently trying to make a magic potion with chemistry.

I want to turn black liquid clear to reveal a hidden message. 

I am currently using water, blue and red food colour and bleach. This works but I want something more dramatic and safe for kids. 

Any Ideas :)? 

regards Daniel
One possibility:  Starch/iodine solution + sodium thiosulfate.  The starch/iodine solution goes from dark blue/black to clear when sodium thiosulfate reduces the iodine.