strongerwoman,
I don't have much to offer, but wanted you to know that I am reading your situation and learning from it. I've pictured myself in your shoes a million times and very well might end up in your shoes. I've wondered about how the custody of the kids gets handled in the interim. Consequently, I've been documenting my husband's drinking/outbursts/etc. for over 10 years. I met once with an attorney about 9 years ago who said she had never seen such thorough documentation. I don't plan to be ill-prepared when I go in front of a judge.
I have an 11 year old who I'll be telling about his father being an alcoholic very soon, like possibly today even. From everyone I've heard talk about it, it was always a liberating experience. I know my son has to know something.
At this very moment in your split from your spouse, you will continue to have to protect your daughter from her own father. And if she doesn't know the reason why, over and over you will look like the bad guy. He'll look like the good guy. Would you rather shatter her image of her father but be able to keep her safe, or run the risk of her starting to resent you for keeping her from the fun parent?
I have a similar situation coming up. My AH's family, mother in particular, is about as looney as they come. I'm not going there this Christmas. And since I'm not going, my children aren't going. That's just a rule I have. My kids don't go out of town with active alcoholics. So, if my kids don't know why, they'll think I'm being mean. If they know why, the blame can go squarely where it belongs.
Think about it. I know, know, know that you want to protect your daughter. But protecting her image of her perfect dad is going to cost you, and cost you more and more as time goes on.