I'm curious about the "illegality" issue. If the company that you work for provides you with a cell phone, everything you do on the phone is company property and they can legally eavesdrop on any phone conversation, text thread, e-mail thread, etc., that they want. Why wouldn't this be the same thing for a parent with the phone that purchased and carries on his account? Don't get me wrong, I would not do this type of thing with my kids, but if I suspected that they were into something that may lead to trouble for them, I might be tempted. I did install LookOut on their phones and told them outright that I may use it to determine whereabouts if I see fit. I have used it one time. My daughter called me just after she got her license and said she was lost. I use the "Find My Phone" feature and saw where she was. After I stopped laughing, I told her how to get back home. She had gotten on the interstate and just neglected to get off. Her brother was with her and made her pull over and call me when he realized (he doesn't drive yet) that it had never taken that long to get home before. She was 20 miles in the wrong direction when I located her.
Anyway, I was just curious.