Windows 7 is the most parent-friendly operating system Microsoft has ever developed. It gives you a great amount of control over your children's computing habits, and can help protect you, and them, from harm. This is done from the "Parental Controls" screen.
To start, left-click on the Start button, then left-click "Control Panel" in the right-hand pane. From the Control Panel menu that pops up, choose "User Accounts and Family Safety". From there, click on "Parental Controls", as outlined in red above.