Kurupira Web Filter is an unusually straightforward and free parental controls package which still manages to include a surprising number of functions and features.
The web filter, for instance, will by default automatically block "inappropriate" content. This isn't exactly fine-tuned, because you don't have to give it the age of your children, or configure it separately per user: it's strictly "one setting fits all".
Still, this also means the program is exceptionally easy to set up. And if you don't approve of the default setting, you can alternatively use it to block specific sites you don't approve of, or to disallow access to all but the sites you specify.
Kurupira Web Filter is also able to set time limits for the use of the internet, or block particular applications from being used on your system.
Alternatively, if you don't want to be quite so strict, you can simply allow your children to use the computer as they like, and just use Kurupira Web Filter's reports to occasionally monitor their activities. These can show you the internet searches they've made, websites they've visited, the programs they've launched and more, perfect if you'd like to make sure they're not straying onto the darker side of the web.