Wow a crazy good idea!! Very good take on the theme! I like how it was implemented, and the random voices from grampa as well as the phrases that would come up are great!
One issue I did have though is it seems sometimes after missing the first key, for whatever reason all keys after that would fail, even if they were the right key (though now that I'm typing this and thinking about it... was I supposed to have him hit a backspace key??) I did find this pretty annoying as it happened on the last phrase a couple times meaning I had to repeat the previous phrases again. If the backspace key was supposed to be hit then maybe showing that highlighted on the keyboard or as a prompt somewhere would help, though honestly I don't know if I'd still like trying to use backspace as that would add extra complexity. I think just hitting the right key again would be more efficient.
Other than the above though I thought this was an extremely good concept that you all made look professional! Very well done to you and your team!!