We use openDNS, and allow our 9 year old daughter to roam YouTube.... But we do monitor her emails and talk abotu some of the nastiness that she's already been exposed to:
1) Her cousin thought it would be funny to send on a porn link that someone else had sent her. Thank goodness it was just a login screen, and the iPod couldnt render the rich flash background that accompanied it on a PlayBook or PC.
2) A chain letter was circulated, containing some vicious and disgusting consequences it it weren't sent on immediately. We had conversations with the other parents (she sent it on because she couldn't get a response from my wife or I within the expiry), and I emailled some debunk links to the hundreds of addresses (bcc) on the chain, reminding them that some of those email addresses are school children. Surprisingly, I only got two snarky comments back.
We talk about healthy choices, and stuff that's on the internet that's appropriate, silly, tasteless, or just plain wrong, and to close the browser if you end up at something inappropriate.
Scary, but they do need to learn self-control at some point... Given some of the conversations that I've heard about from the school yard, Grade 4 is already a scary place.
My 2 cents....