Off the top of my head, I can think of a couple solutions.
The first, is end user training. In this situation, tell your friend to create an account for the kids that do *not* have local admin rights. Now, they still could mess things up, but likely not as badly.
Another 'training' option would be for this friend to find some kind of appropriate disciplinary action. I'm not a parent, so I don't know what that would be...?
Now, for the technical answers I can think of...
> But first, does this friend have a second computer the could install a linux distro on? I'm not saying use that as main os, but to run snort on it. Snort is a damn good IDS/IPS that can stop traffic based on pre-defined rules. 10/10, five stars, two thumbs up kinda software. Also can block based on keywords... if ya catch my drift.
I'm not aware of any parental control programs... but I think Windows included one with their latest bundle of goodies (live or whatever).
Good luck.