A very interesting question, and I'm sure there's a solution, actually. The key is a simple command-line utility included with Mac OS X called screencapture.

Checking the man page for the command (in the Terminal application, type man screencapture) (you can find Terminal in Applications --> Utilities, in case you haven't used it before) (that's a lot of parens, too. I feel like I'm back programming LISP again) (eek!) there are two key flags you want to utilize:

-S: In window capture mode, capture screen, instead of window
-x: do not play sounds

Probably, you also want to point to the main monitor, in case there's more than one screen hooked up, in which case you'll also want to specify -m. The full command should look like this:

This would take a single screenshot and save it in the file "screenshot.png". Now just open up your home directory through Apple's standard file sharing capability and you should be able to refresh the directory occasionally, reopen this file, and you can see what they're doing.

Of course, if you want to go a bit, um, overboard, you could turn this into a shell script that will run every "X" seconds, saving a new screenshot in a hidden directory. Like this:

I've tested this on my own system and it works great. I saved it as "capture.sh" and simply got it started with sh capture.sh &. Thereafter, every "delay" seconds the script took a screenshot, neatly squirreling away in /tmp/.captures for later analysis. If you're so inclined, you can modify it even further, but this coupled with you accessing the system remotely via either command line, ftp or Apple file sharing, you can easily see as many snapshot of what they're doing as you'd like. Remember, though, if you find anything incriminating you'll need to save it on your local system as everything is routinely overwritten as this script continues to chug along.

I should mention that there's an ethical consideration here too. Should you tell your children you are actively monitoring their computer usage? Should you tell them what you're doing and inform them that if they want to use the computer they need to agree to this relatively benign surveillance? This is obviously an even greater issue if you launched this on the computer of a spouse or colleague: then you're moving into 'spyware' and there might well be legal implications. In any case, with your children it's more ethical than legal, but you should still think carefully about whether you want to tell them or not.

Anyway, whether you go for the complicated script demonstrated here or just occasionally log in to their system, type "screencapture" and then open it remotely, you can see that there is indeed a solution to your problem on the Mac.