Q: Managing the imessage on ipod

An iPod (and iPad) on has a phone number for Messaging (and FaceTime) if the same Apple ID is used for Messages and Facetime on an iPhone.

To separate Messages and FaceTime see:

MacMost Now 653: Setting Up Multiple iOS Devices For Messages and FaceTime

If you also want separate Calendars and Contacts and some other things

Create a NEW account/ID for her 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.