I guess I should have clarified why I chose chalk at that time.

I wanted a form of writing that would eventually come off. I'm not too sure anyone would appreciate a permanent form of writing on their sidewalk.

Maybe I chose the wrong place to write it. Now that I think about it, maybe writing on a wall would have been better. It would be more easily seen and less likely to be erased by foot traffic and/or rain, but it could also be displayed without risk of drawing a graffiti fine.

The unique signature or symbol is a great idea. Sort of like a cattle brand or family seal. Something that is easily identifible even by the younger ones in your group that can't read yet.