Because I cannot yell very loud and my children have common names used by many other families, I have developed a special whistle
that lets them know that I need to know where they are. When I walk into daycare, all I have to do is whistle and they both turn their heads
and smile. Once, in a store with my mother, I had my son and she had my daughter, I was ready to go and couldn't find them. I whistled
and my daughter told my mom "I have to go, my mommy's looking for me." My mother hadn't heard anything but little Katie heard me. 
We have used this technique since the kids were very young and it has been a great safety technique for my family.

Lynn , Mom of 2
Florida