My iCloud on my parents iPad

Last year I bought an IPad for my parents (in their 80's) and they love it. I set it up for them when I bought it and used my Apple ID to get them a few starter apps. Eventually, we set up an Apple ID for them and they use that now.

Yesterday they called me and said that suddenly any time they turn their iPad on, they only get a window that asks them to log in to iCloud. It automatically show my email address. They cannot change the email address. If they enter my correct ID password, it tells them it is incorrect. If they hit cancel, the window remains. They can't cancel it, swipe it away, delete it or anything. They are totally unable to access any other window or app at all on the iPad.

I tried turning off iCloud on my Macbook Pro and iPhone, but that didn't help.

Any suggestions or solutions?

Thanks.