If you want people to come back to your workshops, then don't criticize. Nobody wants to be criticized in public, and you don't know these people well for them to trust you.

Not to mention that it's very hard to convey what it is you find wrong. (I was at a workshop once where a teacher tried to convey in several different ways what she didn't like about my playing. None of it meant a thing to me. I finally decided to ignore it all.)

Instead, give them good and bad examples to listen to. It is much easier to take criticism of a third party, especially a third party who willingly did a bad job for teaching purposes.

Then urge the singers to record themselves at home and listen in private. Are they happy with how they sound? In the final analysis, that's what matters.