So, I take it there are no active witnesses left in your family, or they're never discussed (or both)? That is serious progress - that you were JWs and your grandson has no clue of their existence. "Bad church" is definitely a perfect response to his query!
The only one he knows is his great grandmother, my mother, and it never comes up. She would never talk to him about it, so he just didn't know.
My husband's youngest brother and his family are all JW's, but they cut us off years ago. He has recently started communicating again, first with a note with his email address, and then with emails. We'll see what happens. So far they are pretty tame, just catching up with family stuff.