If you have Leopard (Mac OS 10.5), just go to the System Preferences (in the Apple menu), and then to Parental Control.
If you have Tiger (Mac OS 10.4), go to the System Preferences and select Accounts, then select a Standard Account (can't Parental Control an account that can administer the computer) and to the Parental Controls Pill (or tab) and check the boxes for Mail (and click Configure... button to add emails addresses that the user can email) and iChat (and click "Configure..." button to add accounts that the user can chat with) and Safari (and then log back in as that user and go to Safari and add bookmarks to allow those pages to be viewed).
You will also want to lock down the Google search: go to google.com and click Preferences link, select the "Use Strict..." radio button in the SafeSearch Filtering area and click Save Preferences and OK.
Finally, put the computer in the public area of the house, like a family room and not in their own rooms, where some day they will hide and get into trouble. Spend time with your kids and be close by to answer questions and teach them to make decisions (sorry, one day, they will be grown up and be making decisions for themselves, perhaps even by the time they reach 12). You can also go for alert/monitoring tools like McGruff SafeGuard or try AOL's free parental controls. Good luck (i.e. God bless you).