If you click on start, then control panel, you should see in the list of options 'add / remove accounts'. This should let you do what you want. However, if nephew has decided not to allow you administartive rights, you won't see this option when you go the control panel.
BTW are you sure the computer is OK in the girl's bedroom? Kids tend to get themselves in too deep with issues esp. those generated by social networking sites and chatrooms. Heva read:
http://ceop.police.uk/