This is not urban legend. I heard this from a mom who knows the mom and dad in the story.
Family is vacationing in Spain with their four children, 3 older (2-8?) and an infant. Mom is at the hotel with the infant and dad takes the older kids to the park. At one point, panicked, dad notices that, um, he is missing a child. With limited Spanish he grabs the two he still has, runs to a police officer and sputters out "Tres ninos!" and points to his two present children and says "Dos ninos!!" The police immediately understands based on the words and the frantic nature of the delivery. As the officer starts to radio in this missing child report, another family is walking towards him with nino numero tres.
All is well. None of the children are nearly as upset, nor therefore relieved, as the father. Apparently had he not flown into a panic he would have found number three within moments. Dad brought three children back to the hotel. He relayed to the mommy friend who shared the story "There was not enough ice cream in all of Spain to keep them from telling their mother what happened!"
Here's to parents everywhere. We do a hard job. And we sometimes perhaps live in a little more fear than is necessary. The world is scary. No doubt. It is also kind and wonderful and the odds are that the missing nino is nearby.