Sharing an iCloud account
There is actually no other option (unless you want to do some more advanced stuff which probably shouldn't be done on a device you don't use yourself). There is one user (or more precisely one set of settings) on an iPad and you can configure one iCloud email/calendar etc account. You can have a separate account for iTunes and the App Store, but this is probably not what you are looking for.

OTOH, you may want to look into iCloud email aliases to give both your mother and your father individual email addresses. See the answers on Is it possible to have 2 iCloud email accounts in IOS 5? for details.