Thanks to everyone who responded.
It may be that I need two separate kinds of programs to address my concerns/needs.
1. Control - I want/need to have some control over their computer usage (time, internet sites accessed) which is typically what I have found available through parental control software, especially that bundled with Vista or McAfee. We have McAfee through our At&T internet service for free so I have been using that for our firewall/virus protection etc. But I can't install the parental control software on my kid's computers and get it to run.
Long story short, their auntie gave them the computers and they came with a mish-mash of software on them some of which is apparently cracked. 
They have some weird combination of Microsoft XP and Vista as the operating system. I could replace the operating system on each computer but I am not a computer geek and I'm afraid all I would do is make the darn things inoperable.
Of course this may pose a problem with any software I buy...
2. Information control (AKA spying) - I want to know their passwords, what they're saying to each other and other people. I want to block their ability to share personal information like phone numbers and most photos AND I don't want them to necessarily know that I have or can get this information. For this I would need software that tracks keystrokes and provides the option of a much higher level of monitoring that is normally available with parental controls.
I want and need this in part because we have already had issues with the sharing of personal information online with strangers and secrecy over passwords and social networking site participation. Despite the fact that their computers are in the family room in plain site I have found that I cannot be looking over their shoulders every minute - nor do I want to be. I would love to act like I trust them while monitoring their activity in detail.
