Pretty much this. I had people looking for this software every week when I worked at microcenter, and a few wanted guarantees that the kids wouldn't stumble across bad things. Too many times I've wanted to say "bitch, this is the fucking internet."
That being said, because the software sold would of been sub-par at best a decade ago, there is a lot of free goodies I recommended instead. If you're looking for simple content blocking, there is a ton of google chrome and firefox extensions that do a better job than any paid solution. Problem is if the kids are smart enough, they can be disabled.
Believe me, if they are better at doing things on a computer (or quicker) than you, its a matter of time. That's how I was. I was experimenting with linux live CDs when I was 11 or 12. It doesn't get by watchful eyes, however.