I have K9 installed. I know only half of the password, my kids know only the other half.
I have two users on my computer. The administrator user, same thing with password. I know half, my kids, the other half. On that user, we can use the internet, but k9 filtered. And only if both I and my kids enter the password together to log onto administrator.
Everyone knows the password to log onto the other user. On that user we can use everything on the computer except internet. The only site it allows is gmail. Nothing more.