First set up separate User accounts for your children and password protect all of your Administrator accounts, as explained in this JustAskGemalto.com article. Do not let the children know the password, as this gives you control over what they can install and change.
With separate User accounts you can set different Web content filtering rules for different users using Microsoft Content Content Advisor. You can use Parental Controls to set limits on the hours that children can use the computer, the types of games they can play, and the programs they can run. Search Windows Help for "Set up Parental Controls" for detailed instructions.
For more information on Web filtering with Microsoft Content Advisor, see, How do I block selected websites from my child?