Listening to monologues does get real old. I've been there. People like that don't change, either. It doesn't sound like much of an evening out for you, with a 6 year old sitting there. The kid is not going to enjoy just sitting there and listening. Canceling might well be your best bet. Maybe you can go to that workshop instead.

If she asks why you are canceling, you don't have to tell her the truth. Or you can go right ahead and do that. You don't have to get all mad. You can just say that you think it would be better to go another time when she doesn't have another obligation at the same time.