Walt Disney World is a vast place, with four theme parks, huge parking lots, the Downtown Disney shopping district, hotels, miniature golf courses, and much more, all spread out over about 40 square miles.
Normally, your family will confine itself to one park or destination at a time, but it's still possible for individual family members to get lost, or even to lose the car. Here are a few tips to deal with such calamities:
1) Take a photo of the spot where you park your car. Better yet, take a few. Snap a picture of your row number, the sign that lists the section, and anything else that will help you narrow down your car's location. Virtually every cell phone has a camera, so there's no excuse for leaving your car and trying to remember its location with no technological assistance.
2) Teach the kids how to recognize Cast Members. Kids can get lost in the parks, and they often panic when they realize the rest of the family is missing. Teach them what the Cast Member name tags look like and instruct them to immediately go to the nearest Cast Member if they need help.
3) Set a meeting point. It's a lot harder to get lost these days, at least if most or all of your family members carry cell phones, but it's still possible. Phones can run out of battery power or lose signal for some unpredictable reason. Choose an easy-to-find meeting spot in case of emergency. For example, the sorcerer's hat at Disney's Hollywood Studio or in front of Spaceship Earth at Epcot are ideal spots.
These tips are simple and common sense, but use them and they'll save a lot of stress.