Are you aware of who's following your children on facebook or any social media venue?
Do you know the people personally? Where does your child know their social media friends from?
Unfortunately due to the ability of being anonymous in this computer age, it's easy for people to follow your child that do not have good intentions.
This is the age of illusion too, people can pretend they are a young person (even though they maybe an old man/women) by finding a picture on the internet and copying it and using it as their profile picture.
So be a little nosy and find out who your child is following on facebook.