[warrenm]

I'd think "best" might be a tablet (an Amazon Fire'd be your cheapest option), with "just" the one app installed and signed-in

Set the device font to "large" (or whatever it's called thereon), and set the screen timeout to the longest it can be (but autoshutoff is still a Good Idea™)

Maybe enable parental controls on the device to only allow that app and/or not allow the app to be swapped-away from?