I have a colleague with two teenage boys (yes, cue cliche's about 'I have this friend see...') He's currently having issues with them browsing pr0n and wants to do a little spying on their browsing (I'm staying clear of the philosophies/ethics on this.) The kids are savvy enough to clear their browsing history when they're done. As I'm his goto for IT he has asked me if there is a way to keep a hold of the browsing history.

The family uses Macs, and the kids surf with Safari. I know that browsing history is kept here ~/Library/Safari/History.plist. I figure there should be a way to write either an AppleScript or other script (Python/Ruby/Bash) that can backup this file to a different location (/opt/local/history, etc.)

Since the kids know to clear their history when they're done should the file be periodically backed up with something similar to a cron job or something like Hazel? While that could work it seems like it would create a ton of little incremental backups.

Or is it possible to 'watch' ~/Library/Safari/History.plist and incrementally add changes to a backup file (saving a diff so to speak) but not lose any data?

Any ideas/solutions appreciated.

UPDATE/EDIT: Got the word from concerned dad that the oldest uses Firefox on a different PC, so the OpenDNS solution (preferably at the router level) is the best answer so far as it would capture usage for the whole house.