LOL - very helpful Glen - but a little late ya think
Andy - welcome to the site
I'm not aware of anything even resembling what you describe that would restrict access to only parts of MCE,
You may have a light at the end of the tunnel however once the xbox 360 lands, assuming you have it hooked up in another room, your kids will be able to play watch live tv, recorded tv, play music, etc. using the xbox without accessing the internals of your media center box.
Might be a solution?