Have you ever talked to your daughter about dad's drinking before? Or has she ever talked to you?
I'm not a mom, but I was that daughter once. No one ever talked to me about Mom's drinking. I just thought she was angry all the time and that it was normal. I thought tip-toeing around her and trying to be perfect so she wouldn't explode was normal. So I cannot give you advice from the parent perspective and I wouldn't dare to say what is age-appropriate, I can only tell you that I wish someone had decided NOT to keep it a secret, or assume I understood, or that I wasn't affected or whatever. I wish someone had told me it wasn't my fault and there was nothing I could do about it, and that it really didn't have anything to do with me. I am confident others will be along with better advice about what to tell your daughter.