This is not only a good idea for keeping your kids tamed, but also making sure that anyone else who is not authorized messes with your computer. Personally? I have password protection on all my gizmos, from my Apple iPhone 3GS to my Mac systems to my PC computers. An ounce of prevention, as they say...

The trick is that it's not obvious where on a Mac you find this option. Initially, you might have looked in the "Screen Saver" settings, but that's all about what to show when your computer is idle (though on a PC that is where you can find the 'require a password to wake from sleep' option there, though in Vista it's "on resume, display logon screen").

Instead you need to look in the "Security" area.

Let me show you. First off, click on System Preferences... off the Apple menu on your Mac system. Among other options, you'll see:

Now, click on the Security icon and:

You can see the option front and center, and you can see that I've not only checked it, but also specified that it only kicks in after a minute of idle time. This way if I'm working and, say, on the phone, if the screen saver kicks on I can touch my mouse (or trackpad) and go back to what I'm doing.

If that's insufficiently paranoid to avoid your kids causing trouble :-) then you can also specify "immediately", coupled with tweaking the screen saver settings to have it start up after just 2-3 minutes of idle time. Between them all, assuming you have an unguessable password, you should be safe from their foraging fingers!