If the child is well enough versed in computer and know where the modem, the option to disable it just does not fit. Because the child will come to the modem, you will see that it doesn't work, because it will illuminate the corresponding indicators, and just to turn it on. So turn off Internet programmatically, directly in Windows.
There are different ways of disabling the Internet for Windows XP and for Windows 7. In Windows XP go to "control Panel" located in "start menu". In the list that appears, find "Network connections". If your Internet connected via ADSL technology, i.e., every time you make a connection to the server, entering username and password, then click on this connection right mouse button and select "Disable". The Internet connection is disabled, i.e. no access. But if the child knows how to connect to the Internet, then this will not be enough. Disconnect the modem itself. Normally, it is named "local area Connection". Click on this link, right-click and from the menu select also "Disable". Now it's time to connect to the Internet it becomes absolutely impossible because the system doesn't see active connections and does not know where to make requests. The inclusion of the modem is carried out in the same way (PCM to Turn on and wait a few minutes until the system is properly configured and defined).
If your computer has Windows 7, something similar to: open "control Panel" located in "start menu". Then in the proposed categories, select "Network" and "Internet". Now three of the provided items you will need first – "network control Center". In the left menu select "Change adapter settings". Now you'll be in the same "Network connection" as on Windows XP. Further the algorithm of actions similar to that described above.