How to ..
As the internet is a sort of unlimited library containing all information to be accessed by everyone — in contrast to the previous century's paradigm of information being difficult to locate — it follows that everything can be found if you search for it. That includes less savoury information such as pornography and other material unsuitable for children (and adults!).
Apple recognise this and have built in a feature to control the information available to your kids called Parental Controls.
Although this is simple to switch-on and configure a couple of things need to be made clear. It relies on the fact that the child has a separate account on the computer which needs to be a Standard account not an Administrator account and that the password for your Administrator account is kept from the child.
For information on how to set up accounts see Adding and Deleting User Accounts. Anyway, its a good idea to have seperate accounts for each user of the computer.
While logged into your usual account after setting up the child's account go to System Preferences/Accounts and click the Open Parental Controls button.
Then click the next tab called Content:
If you click the box Try to limit access to adult websites automatically you have a further Customise option which lets you literally add and delete what websites they can visit. In practise this is very difficult if not impossible to achieve.
Its a matter of trying your settings and then checking back regularly to see if it's working along with monitoring their responses to what you have done. Each kid is different and many will be wailing about not being able to go to certain sites they could visit previously!
Next is the Mail & iChat tab which really is just for email via the Mail application if you wish to limit who they can send and receive email from. I find in practise that this area is usually ignored by most people:
Then a last tab which again not many people bother with which is a log of all the places the user has visited online.