Yes, happend to me as well. Crowded vacation spot. I told my kids that if they get lost to go to a police officer, store owner or to a mom with children. (no mom could ignore a lost child!). That is what my son did, he found a mom. Worst 5 minutes of my life and the what if's were torturous.
That said... in a crowd, we now often write our mobile numbers on their arm under their shirt in pen or sharpie. Also, we pick out a landmark. If you get seperated from us, go here.
I learned somewhere to have a recent photo with height and weight on it. I carry those in my wallet. (I'm not good at remembering their stats). I had that picture in my hand when we got seperated from my son. You can also take a pic with your mobile phone so you have current clothes or make a mental note of what they are wearing.
It happens to everyone at some point! Thanks for starting this posting!