All you need is a fine point sharpie or sharpie-like pen, write "if lost call" and your phone number, then put some liquid band-aid on top, let it dry and you have a water proof temporary tattoo that should last for about a day. A California police department posted this tip on their Facebook page and it's been shared thousands of times.
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If your handwriting isn't great, you can get some Temporary Tattoo paper and print your information off (make sure it's backwards so it shows up right when you put it on) and then put liquid band-aid on that after it's transferred.
The Clovis Police Department also recommends you take a photo of your child using your cell phone the morning of the event so you have their clothing, hair style, and up to date picture in case you're separated.