We're not sure if this is a little devious, but there are several apps allow parents to track trick or treaters.
Track n Treat .. free iPhone app which allows children to send a time link of their location. Parents can then track them on the computer
Family GPS Tracker .. free for iPhone and Android users .. alerts get sent when kids venture outside certain areas
Life360 ... free app for iPhone and Android users. Allows family members to view each others' locations on a map