You can use iptables's owner extension to block a user from accessing the net. Open a Terminal by pressing Ctrl+Alt+T and enter:
sudo iptables -A OUTPUT -m owner --uid-owner user_you_want_to_block -j REJECT
user_you_want_to_block with the actual user ID of your daughter. As long she does not have administrative privileges, that is, can't use
sudo she won't be able to reverse the change.
See How to restrict user's access to the internet for time intervals?
This solution will work until you reboot the computer. Once restarted you will have to issue the command again as the administrator. There are various ways of automating this described in detail at Ubuntu iptables help.
Hope this helps