M.,
What a sweet time in your daughter's life. We homeschool our children as well, and I agree....she probably still believes because she isn't exposed to "the world" like other children her age. I would wait it out through this season, but I would definetly tell her the truth before next year. She will probably find out from others and it would be better to hear it from you and your husband.
We told our children and took some of the sting out of it by telling them the true reason for Christmas and the season. We celebrate Jesus' birth and the giving of His life for ours at Christmas and our children have never looked back.
The truth is you are probably reaaly facing the idea that she is "losing her innocence". That is a tough thing to face and with so many things to deal with in this life it is hard to let them move toward being grown up. I can definetly feel for you there. My son just turned 15 and got his driver's permit......ohhhh do I feel old. (Wasn't it just yesterday that he was in preschool being a lion for the circus play?) Our daughter is almost 13 and everyday I realize she will be a full grown woman and there is nothing I can do about it.
Hopw all goes well, Merry Christmas and Happy New Year!