This can be done in software a number of different ways. Changing the IP address or gateway, downing the wlan interface, preventing the wifi driver from loading (or unloading it later).
The best way would be to change the firewall settings to prevent wifi traffic from getting in or out (iptables). Read about how iptables works.
If you disable wifi by learning how iptables works, then you will know how to enable when you want to. If somebody gives you a quick fix, then how will you turn it on when you need it? Give a man a fish or teach him to fish? That is why linux geeks are so fond of answering many questions with RTFM -- it means it would be much better for you to go learn it before messing with it...
Of course, if your daughter becomes a linux geek herself, you should disable wifi multiple ways besides iptables to delay when she "hacks" it back on.
Last edited by geekmaster; 01-06-2012 at 04:57 AM.