All the major carriers have been offering real time tracking for a few years (or longer) now, so whatever company your plan for her phone is with, you can contact them for more information. Most "dumb" phones are supported, as are all smart phones, but pre-paid phones might not be depending on the carrier.
Oh, and she'll probably have to be part of a family plan for you to track her phone.
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I like this feature with the iPhone. You can register all of your iPhones, iPads, iTouches, iWhateverthehellitis on an account and you can track on a map or if you cannot locate it, you can send an audible alert to the device even if it's on vibrate to play if you have difficulty finding it. Additionally, if the device is lost, you can do a remote wipe to keep your stored information from being misused.
This was available previously through MobileMe at a cost, but is now free through iCloud.
edit: Don't know that this feature works if the phone is off though. She's a teen though, so it's not like that would ever happen.
:-)
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