Here's how to do it. When she's not around to watch over your shoulder, go into the "Settings" app. of your iPad.
Then open "General", followed by "Enable Restrictions".
This should be password protected by a 4 digit number,
different to the opening page number. Set this password number to something she doesn't know (but make sure you can remember it!).
In this page, there is an option to stop many of the features you are worried about. Just slide the buttons next to "Installing Apps" and "In-App Purchases" like the picture below. You can bar the "iTunes" store too here if you want. Depending upon her age, you can restrict access to explicit content too on the same page.
Come out of "Settings" and you and she are safe.
By going back to "Restrictions", you can change these settings at any time to let her download under supervision; just don't let her see the password number! Hope that helps.
see Apple's advice on this too:-
http://support.apple.com/kb/ht4213Chris