If all they've deleted is the history then you need to check the cache and the cookies. If it's IE then you can click Tools/Internet Options, click the Settings button and then click View Files. Click on where it says Internet Address and that will sort the list (which may be very long) by the site that are cached. If you move to the right on that window you will see the last accessed date. Depending on the software on your computer you should be able to view the cached photos to see if they're visiting porn sites if that's a concern. If they have cleared the cache then there is little you can do for now. You want to control the sites they access then I recommend
Cyber Patrol