That's interesting, in that there's a risk that, if they feel really unsafe, they'll want to pretend that all is fine

And if the group is small, even if you try to make the answers anonymous, maybe you still would be able to figure out who wrote what, and they'll realize this, and write mostly nice sounding things

Anyway, asking and talking about it seems like a good idea. Maybe also asking them to read the article being discussed here :-)