Hi Garce,
Here's what you can do:
1st. You can see the usage by clicking on SETTINGS / GENERAL / USAGE / CELLULAR.
2nd : The iPhone lets you turn off Cellular data. Doing this will force it to use only your wifi connection.
3rd : Make sure that "Personal hotspot" is turned off. She or others may be tethering to it to connect to the Internet via the Cellular network which can be expensive.
4th. Use parental controls to lockout the changes :
http://www.demogeek.com/2009/07/01/r...-restrictions/
There's also these and other apps to control data usage:
http://dailyiphoneblog.com/2010/12/2...es-data-usage/
Etc etc etc.