One of my daughter's best friends in Highschool was the child of a white German mother and a black American officer who had been stationed in Germany. They fell in love and married, and when his deployment was over, he brought her back to the states where they had five kids, none as dark as "Dad" but none as white as Mom, either. The Dad died shortly after the birth of #5. For whatever reason--perhaps she thought her mixed race family would have a better shot in the USA--she decided not to return to Germany and she has raised her family here. She and I met and became friends while participating in the kids' school activities.
Her answer to her children was yes you are white and yes you are black. And that makes you lucky. She made sure they knew they were smart and loved and special and beautiful. And she made sure that they knew what they could be proud of in their black and white heritages and their American and European heritages.
She confided to me that she did feel somewhat left out socially at times and felt that she sometimes didn't really belong in the earlier years. She didn't know if it was her heavy German accent or if it was because she was single and raising a brood of mixed-race children in a very "white" town. I suspect it was because she was a single mom with a large family in a very traditional 'couples' town, but at any rate that all wore off in time and she became quite Americanized. She is now a committed Kansan who is enjoying her grandkids a lot.
Her kids grew up quite well adjusted, took part in drama and music and sports, and so far as I know have done quite well for themselves. They would all be in their 30's or 40's now.
So, I guess my advice is to help your daughter believe she is the most special person in the world, is well loved, can do anything she wants, and help her know that it is okay to be 'different'. If she doesn't know what to say when kids ask her about her race, she can say "I am black and white". Aren't I lucky?