Back in 1994, in an essay called "Why Censoring Cyberspace is Dangerous and Futile," Howard Rheingold gave some advice that is still relevant today:
Sooner or later, our children will be exposed to everything we have shielded them from, and then all they will have left to deal with these shocking sights and sounds is the moral fiber we helped them cultivate.HT: Tom Donovan
Teach your children to have no fear of rejecting images or communications that repel or frighten them. Teach them to have a strong sense of their own personal boundaries, and of their right to defend those boundaries. Teach them that people aren't always who they present themselves to be and that predators exist. Teach them to keep personal information private. Teach them to trust you enough to confide in you if something doesn't seem right.