An a single iOS/iPadOS device can create a maximum of three AppleID accounts. This limit has been reached on the iPad that you have.
Your Grandma will likely find this information helpful...
If you have forgotten your AppleID or associated password, they can be recovered here:
https://iforgot.apple.com
To recover your credentials, you’ll need access to one of the following:
- Your primary email address mailbox that corresponds with your AppleID
- Any of the secondary/recovery email addresses that should be configured for your AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID
Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.
More information about recovery of your AppleID:
If you forgot your Apple ID - Apple Support
More information about recovery of your AppleID password:
If you forgot your Apple ID password - Apple Support
When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:
https://appleid.apple.com
If you intend to reuse the iPad with a different AppleID, you would be well advised to prepare it properly for reuse - as this will ensure that the Activation Lock associated with the original AppleID is removed, along with all data and other sensitive information:
What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support