Demos, paid or un-paid, are not legal, but they are also standard practice. IMO, it sounds like the woman who “interviewed” you was just trying to line up a sub for a teacher who may be out.
I’d give David’s a pass anyway. I taught one adult class at a David’s in Koahsiung once and they were teaching adults the same kids songs, and using the same kids books, as any other bushiban. Of course, that was years ago. Anyway, surely you can find something else.
But you may have to do a demo. Yes, it shoud be short.
Good luck.