I took my parents on holiday to Egypt many years ago for their 40th wedding anniversary, flying Egyptair. I upgraded us all to business but didn't tell them until we got to check in, wherein the game was up by virtue of the queue we joined.

They loved the experience, and the surprise.

As I was travelling with them, it was fine to not give pre-warning. However, I ditto the above and would at least let them know a few days ahead of the trip so they know what to expect and can make the most of it. Tell them to tell the crew it's their first time and it's a surprise treat so they get extra attention!