When you go to an event, the store or anywhere with your kids, you risk them getting lost. It only takes a second for them to run off or dawdle behind. I remember one time Noah and I were at the grocery store. It was shortly after we got our kitties. We were in the pet aisles. I had finished getting what I needed and kept shopping. About five or so minutes later, I noticed that Noah was not with me. I thought maybe he was with my mom but hen wasn't. He was 9 so I wasn't took terribly worried. I went running around looking for him. He was looking at the cat toys. Noah is now old enough to give out my number or state if he's lost. Younger kids don't have the skills to do this. There's all kinds of safety things out there from tattoos to bracelets. I would have no qualms taking masking tape and marker and sticking it on his shirt.
We go to a lot of festivals and fairs. We seem to have a lot of the plastic band bracelets. We have them with all kinds of logos. Noah got one that says Shark Con 2017 because I liked them on Facebook. I have seen ID bracelets with information engraved in the plastic. I wonder if the information is visible or not.
Either way here's a mom hack. Take a plastic bracelet and turn it inside out. Write your info and allergies on the inside with Sharpie. Put it on your kid's arm and draw a small arrow on the outside. If someone finds your child, they know who to contact. I have seen many children wandering around at the store. Save your money on spending $10 and make your own with the free ones you picked up at the fair. Kids love to wear them anyways.
Here's a bonus mom hack using the same silicone bracelet, use it as a drink label for water bottles. You can assign colors or even write names and keep them tucked away for the next time.