Change email address on ipad

Presumably you refer to a shared AppleID - this being identified by an email address.

Sharing an AppleID between users is a very bad idea. Your granddaughters should each have their own unique AppleID - in fact, every user should have their own AppleID. This is essential to maintain intended separation of activities and to avoid unintended sharing of unintended information and data.

You may find this information to be helpful with setting-up an AppleID for a minor - and in considering appropriate controls while continuing to share Apps and paid Apple services if you should choose to do so:

Apple for Kids - Official Apple Support

Help your child set up an iPhone, iPad, or iPod touch - Apple Support

Family Sharing and Apple ID for your child – Apple Support

Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support

The best method of sanitising an iPad is to properly erase - and then setup from scratch with the user's own AppleID. If instead the iPad is already in use, it is possible to sign-out and sign-in with a different AppleID - however, in so doing, all data stored locally and in iCloud (including Photos, Notes, Messages, Contacts, Calendars, Reminders, Passwords etcetera are potentially retained.

If you wish to change and use an entirely different AppleID without erasing the device, first ensure that you have disabled the Activation Lock that will otherwise cause difficulties in the future. You can disable the Activation Lock by disabling the Find My service from iPad/iPhone settings before trying to use a different AppleID account:

Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF

Having disabled the Activation lock, you can now sign-out from the existing AppleID:

Settings > [Your Name / AppleID] > Sign Out

If you wish to preserve iCloud data that is locally stored on the iPad, during sighout accept the opportunity to keep a local copy of the iCloud data (e.g., Calendar, Contacts, Notes, Keychain Passwords etc.) - as otherwise this data will be wiped from your iPad as you sign-out from iCloud. 

Also ensure that you have signed-out from Messages and FaceTime:

• Settings > Messages > Send and Receive > [AppleID] - scroll to the bottom where you will find your signed-in AppleID highlighted in blue text; tap and select Sign Out

• Settings > FaceTime > [AppleID] - again, scroll down the page to find your signed-in AppleID highlighted in blue text; tap and select Sign Out

After signing out, a restart of your iPad/iPhone is recommended:

Restart your iPad - Apple Support

Force restart iPhone - Apple Support

After restarting, you can sign-in to your new AppleID account from iPad settings - and also sign-in to Messages and FaceTime if necessary. Any iCloud data that you saved to your device during the previous sign-out will be merged with your new AppleID account.

By email address, do you mean she shares just the email account? So both receive their email messages to the same account?

Or are you referring to a shared Apple ID being used, that uses a particular email address?

Email addresses in Settings ➜ Mail ➜ Accounts can be added or removed without affecting any other data in the iPad except the email being received.

Can you clarify what it is your granddaughter wants to accomplish?

have you read

Change your Apple ID - Apple Support