I recommend that you
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.
Use the new ID on her iPod but only for:
Settings>Messages>Send and Receive
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.
If you want to use here new ID for purchases note:
- Apps are locked to the account that purchased them.
- To update apps you have to sign into the account that purchased the apps. If you have apps that need updating purchased from more than one account you have to update them one at a time until the remaining apps were purchased from one account.
Also note the 90 day time period specified here. It concerns some iCloud services