Yesterday my daughter's preschool class went on a field trip. They were getting ready to go just as I dropped her off, so I noticed this hack that they use: Before they leave the school, each child slips on a rubber bracelet (of the ubiquitous color-for-a-cause variety) bearing the name and number of the school. It's subtle enough not to attract the interest of strangers, but would serve the purpose should a child become separated from the group. And of course, the kids wear them without a peep of complaint.
The hack for parents: Order a batch with your family name and phone number and use for trips to the zoo, amusement parks, airports, etc. I found a vendor on eBay with no minimum order (although it's unclear how much they cost).