First, create a separate account for your children with the minimum number of rights. In the "children's" accounts limit the ability to delete and install programs and create additional Internet connections and change the settings that I have at the moment. Remove the connection if it is. If you connect to the network using dsl connection, set connection to the computer only for the main account. If you are using a wi-fi router, set a password and disable its preservation in computer memory, and automatic connection. Don't forget to turn off the computer when you leave home.
You can also limit the access time to the Internet using special programs, for example, Kaspersky CRYSTAL. A feature of this, is that you can set the exact time at which Internet access is prohibited, and users, to be subject to this limitation. The easy option is also the setting of the password is guaranteed to protect you from changing the settings and connect to the Internet in your absence.
Often the level of computer literacy of children often far exceeds the level of knowledge of their parents on this subject. There are ways to bypass the administrator password that will give the child the ability to connect to the network. However, this method can easily be seen with the help of the administrator password is not cracked, but simply removed without the possibility of installing the same, that will be easy to notice. Alternative to software techniques is the blocking insulation of equipment with which you access the Internet, modem, router or cables for power and computer connection - from the child in your absence.