Find My Friends for iPhone and iPad lets you set an alert for when a friend (or child or coworker) arrives at or leaves a location. To set it up, tap the perform in your friends list, then tap Notify Me.
Yes, there’s a total creep factor here, which is why Find My Friends gives you so much control over who can see where you are, and when (check the Me tab). I’m sure parents use this for all sorts of legitimate (and sometimes not-so-legitimate) child tracking. Occasionally I use it to alert me when my wife leaves work to catch the train so I know when to start dinner.