Hi , you just need to login in to the Mac as an admin user and make the following settings .
Setup parental controls for the Child's login or user account .
1 Go to Apple menu , System preference , Accounts TAB .
2 Open the pad lock , enter your admin password XXXXX
3 Check enable parental controls.
You can only do this for Non admin accounts.
If the Child's account is an Admin account , you will have to remove the Administrate this Computer setting found in Account control panel with in the Child's account settings.
You will also need to setup your own admin account if there is only one Account on the Mac.
4 Click on "Open parental controls "
Review the Parental controls , they are quite comprehensive .
Read more below in this handy guide:-http://theappleblog.com/2009/01/13/kid-proofing-a-mac-with-parental-controls/
I hope this helps .