Spamers often use text messages(SMS) to contact us. And the content of these messages could be "too adult" for your teen:"How about to connect to a sex-chat or visit a shop of light drugs?" You can monitor text messages on your kids phones with mLite app to protect your child. Try mLite for free for 7 days.
We have found several articles about secret codes in SMS for parents and a kid. For example, if kid send a "X" it means he in trouble right now. But there are more complicated solutions. Have you ever heard about Panic button? You can add it to the main screen on your kid's phone and get an alerted message with precise GPS location of your child straight to your phone. You can try mLite Panic Button