Two options:
MacMost Now 653: Setting Up Multiple iOS Devices For Messages and FaceTime
or
Create a NEW account/ID using these instructions. Make sure you follow the instructions. Many do not and if you do not you will not get the None option. You must use an email address that you have not used with Apple before. Make sure you specify a birthdate that results in being at least 13 years old
Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card
More details of how:
http://ipadhelp.com/ipad-help-tips-tricks/how-to-get-free-apps-from-the-app-stor e-without-a-credit-card/
Use the new ID on her iPod but only for:
Settings>Messages>Send and Receive
Settings>FaceTime
Settings>GameCenter
and Settings>iCloud if you want her to have separate Contacts Calendar and some other things.
Continue to use the same/common Apple ID for Settings>iTunes and App stores so you can share purchases.