I was always the most critical of these threads when it came to marking up your kids, etc when going to a theme park. After all, 3 kids in and we've never had an issue. Then, last year at a water park in NJ, as we strolled off in different directions, my wife assumed that I had my daughter next to me while I assumed that she had my daughter next to her. We were only separating for about 2 minutes, running another kid to the bathroom while the other went to the towel bag to grab something, and in the process my 3 year old was somehow missed. When we both met up again and quickly realized that she was not there, I cannot even begin to explain the degree of panic and fear that sets in. It only took about 15 seconds of looking around where we saw her sitting on a step, teary eyed and looking for us. Again, 3 kids over the course of 10 years and not one issue ever losing, misplacing, forgetting a kid. It took once "hiccup" where assumptions were made on both ends and all of a sudden you have a missing child.
Mark up the child, tattoo the child, even tie one of those really long balloons to her arm, the kind you find at a used car lot. It's all good.
Also, never make assumptions!