You should probably set up multiple user accounts for your computer so that your kids can set things up the way they like. And you can also use Parental Controls to deny them access to iChat.
System Preferences> Accounts> Parental ControlsBut I can't see how to deny them access to specific applications.
System Preferences> Accounts> Parental Controls
Tick the boxes next the application in question.
Hmm... that lets you limit who they talk to. Not sure how to limit the apps...
Hmm... that lets you limit who they talk to. Not sure how to limit the apps...
I guess you could limit them to just talk to you (their parent...which would effectively mean nobody since no two users can be on the same computer at the same time...).