I agree with some of the others about someone cleaning up their tracks.
Especially if your kids are old enough to be slick.
They can be deleting the history file, deleting the autocomplete of content and passwords, cookies, etc.
One tell-tale sign is if your history is all of a sudden empty. Most kids are thorough enough to re-populate the history with the commonly visited sites. Hitting “control + H” brings up History in Outlook, or you can use the dropdown url bar.
there, fixed!