Honestly, I don't think you'll have to worry too much about what your child sees/hears on the internet -- they'll get it from their peers. I got to know *way* more then my young nephews do now, before heavy use of the internet and online gaming, through friends at school and on sports teams.

What I do look out for are viruses and personal info -- they don't know a phishing scam yet so they stand a high risk of getting caught in one. Still, simple enough rule that they don't give out anything without permission.

Over-protection can be as harmful as no protection, just keep a balance and expect your kids to learn about 'mature' things at some point. Likely earlier than later.