My 6 year old is getting good at doing stuff...I'm concerned about this sort of thing as well.
The internet is full of bad stuff. I don't want him on it at this age basically at all.
I'm considering locking down my out bound traffic on my router and requiring authentication for anything other than things like
Wikipedia and
PBS KIDS: Educational Games, Videos and Activities For Kids! just in case.
When they get older and I have to allow them to more I would probably put some sort of logging in place so that I know exactly what they are searching and where they are going. ZERO unsupervised Internet access. ZERO.
So the only thing I would be going after in the logs is stuff they weren't supposed to be doing.
My kids are going to hate that I am a Network and Systems Security junkie.
Muahhahahahahahahahahaha!