Go to SETTINGS > GENERAL > ACCESSIBILITY
Turn on "Guided Access," set a passcode, and turn on the Accessibility Shortcut switch.
Then, launch an app for her to use and triple-click the home button. She will be locked in that app until you triple-click and enter your pin code.
This is even better than just locking the settings because it keeps her from texting people, erasing contacts, returning emails, etc.
You'll have to switch apps for her when she wants to, but that's a small price to pay.
Also, I suggest turning this on when you see her and off the rest of the time. I find that it slows down double-click response a little because it's constantly waiting an extra half-second to see if you're going to triple-click. When you turn it off that lag goes away.