When my family went to Montreal on vacation in the 1970s I spent an uncomfortable few days with an itchy piece of paper stuffed inside my sock. My mom, worried that I would not be able to communicate with the French-speaking population if I got lost, had written down the contact information for our hotel on a piece of paper and stuck it in a safe, though annoying location. This memory came back to me when we took our toddler up to Vancouver B.C. (his vocabulary and grasp of English astonishes me but it’d be useless for a stranger). I didn’t stick anything on him but I thought it’d be neat if you could order a custom temporary tattoo with your cell phone #. Of course I’m not the first to think of it. DaddyTypes linked to a temporary tattoo for kids that has a space to write in a number (it comes with a pen). You could also buy custom tattoo printer paper, assuming you trust that your printer ink is safe for this purpose.