Trish's variation on a hack for lost-kid preparation at amusement parks is so simple it could apply to all sorts of travel situations. I had to "promote" it from the comments so everyone could benefit.
I take the picture of everyone standing in front of the sign where we've parked (we did Disney & Universal this way), so I have a picture of the kids & I can find the car too!
You could use this hack for airport parking lots, shopping malls, or any large parking lot (think IKEA or another big-box store). For backup, take the picture using your cell phone and send it to the other grownup in your group. That way, if your phone goes missing, you're covered.
More: Family travel hacks