I'm very sorry you caught your wife in a cheating affair and she called it "polyamory." That isn't ethical polyamory to me. That's plain ol' cheating. I'm sorry you deal in this.
Cheating on agreements can happen in any shape relationship -- mono-shape or poly-shape.
Peace too you all. I hope things work better for you as I wouldn't wish any of what/s happened/ing to me on anybody.
I wouldn't wish a cheating affair on anyone either. Again, I'm sorry you are dealing in this kind of hurt right now.
You mention therapy for daughter -- I hope attending family therapy is local support in the way that you both need it right now.
I hope over time you and your child can find more ways to express appropriately to safe, local people and heal from this experience.