There's an undocumented (
AFAIK - I can't find a reference) bug in IE5+. Visiting certain websites will cause your text size to be reset. I suspect it's something to do with malformed style sheets since it doesn't seem to effect Netscape. Another reason I still stick with IE4 at home.
Anyway, I wouldn't necessarily blame the kids since this behaviour is imbedded somewhere in the system. Or 'by design' as Micro$oft might say <img src="smile.gif" border="0">