I would not go for a topic, I would have a selection of varied questions:
Snow White and the ???? dwarfs.- Names of dwarfs?
Prime minister's name? - What political party?
What was George Formby for ever cleaning? - Can anybody sing the song?
???? Dalmatians? - do you like dogs?
How many pence in a pound? - When did we go decimal? Can you remember 'old' money?
Can a Bombay duck swim? - What is/'have you eaten' Bombay Duck?
It's all a bit of fun to involve people with varying cognition
I would try and turn each question into a light hearted debate.
Its good you are doing this. I have witnessed quizzes where dementia sufferers get very upset because they cannot remember the answers. It is worth you having some backup material, so you could spend some time talking about the question.
A simple example if you asked a question about Winston Churchill, then you could have some pictures of him with his cigar to prompt some discussion.