The accidental deletion of files is a best case scenario if they surf the net. As I'm sure many of you are aware, typing something as innocuous as "dog" into Google will result in - well, quite...
If you let your kids use the PC, make sure they're supervised, especially if it's hooked to the net.
Don't go overboard though. Let them delve into the innards. How do you think I learned all about this computing malarky? If you have backed up that win.ini file, why not let them see what happens if it disappears?
You'll thank me when your little one has a high paid job in IT consulting. I'll be wanting a cut of their wages, by the way...
Edit: Decided that I should actually type in "dog" into Google. Just for research purposes, you understand. At least the first 50 pages of results is non-pornographic, which tells me that very few people have the common-sense to own cats...