parental control software: advice please!

[#1]

My 11- and 8- year old children frequently go on-line; my computer is in the living room and I do keep my ears (and eyes) open, but would feel more comfortable knowing that they're not going to stumble onto anything undesirable by accident. Once my son goes to secondary school he will be spending time at home on his own for an hour or so after school as well. Any recommendations of free parental control downloads gratefully received!

[#2]

If you using Windows Vista it includes a load of controls for restricting children's usage, if your not then I am afraid I am not much help.

[#3]

Thanks Arthur! But I don't have Windows Vista, I'm a Mac user...if any Mac people have ideas bring 'em on...
PS I'm also an internet learner (but am doing my best) - so if it's simple, so much the better 

[#4]

Its not a simple as you may think as what do you restrict? Often things will be picked up by some parent control software thats ok. Like a site may have reference to sexual part but be in fact a news site or an educational site.
The first and simple step is to goto
http://www.opendns.com/start/mac.php
and setup the Mac to use opendns. Sign up its FREE  then look at
http://www.opendns.com/support/category/145
as it lets you stop adult content. The best of this approach is its the dns that blocks the sites so a child wouldn't know how to change the dns server. 
the next line of defense is to use a FREE proxy server for the MAC like http://www.lavasoftware.net/en/content/ezproxy/features.htm
this again lets you filter the content and most let you keep a log so you can see what the child is trying to look at as well

[#5]

thanks samuria: that is all really helpful I hadn't thought carefully enough about exactly what I don't want the children to see; I just remember suddenly finding some horrible stuff on my computer at work, just a keystroke error obviously, but so easy to do... so thanks again for the great advice.

[#6]

You can also set your google preferences so that unsavoury sites don't show in google search results.
goto google.co.uk and click on preferences.
scroll down to safesearch filtering and select the strict setting.
save the change.