App to keep track of daughter?

My daughter is 12, and she has Down's syndrome. Sometimes she wanders off. And I hate the fear that it installs in us all. You think she is with a friend, and you go to pick her up and they said she said she would walk home two hours ago… Or you go to the toilet and she has gone out the door when you return. (Yes we have tried locking the doors, but then there are the windows… ) and we just want to be able to find her, not keep her from being a child. She functions in what little trAffic there's around here and is not in more danger than other children.

Is there an app I can install on her Samsung phone that allows me to track her whereabouts with precision when she forgets that she is not alone in the world? And, no I can't just call her as she doesn't seem to hear the phone when wandering.
The app I have on my own iPhone to track it with in case it is stolen demands my iTunes password to be activated and I do not want anything like that if I can avoid it. In respect of her privacy, I want something I can install on both phones and put in a password that gives me her location, but not allows me to see what else she might have on her phone. She is 12 and has done nothing to deserve loss of privacy. It's only so we can find her when she wanders off… The alternative is to lock her up and that is not an alternative at all.

A parental control software like mobile spy may be helpful to you: http://www.spy-mobile-phone.com/