Disney has a good security system. The Cast Members know to recognize the signs of a child looking for parents or parents looking for a child and they will offer help.
If a Cast Member finds a lost child they will take the child to Disney's Baby Care Center to wait for the parent to come.
When my family is in Disney the very first thing we show our children is what the official Disney Cast members are wearing. We tell the kids if they get lost to go directly to a Cast Member for help. When my kids were very small I would write my name, my child's name, our hotel name & our Mobile phone number on the inside of their shirt hem or on the inside of a bracelet for them to wear and tell them to only show this to a Cast Member when lost.
The second thing we do when coming to a Disney park is locate the Baby Care Center on the Park Map. It is marked by a Pacifier on the map. We go there to show the kids what it looks like and make it our family "meet up space" in case we are all separated that day.
Baby Care Centers are wonderful for other things too, like changing diapers & feeding small ones!
It is a real fear losing little ones in big crowds, but at least Disney World is experienced in situations like these and know what to do!
It was great to hear from you again! I hope you have a wonderful trip!