I don't really know, but wish you luck (& patience with yourself) in telling her.
Megan-39, Postpartum Doula, DW to Sacha-40 (18 years together) and Mama to Finn Alexander born 4/2/07 and Zivia Littlewood born 8/23/10
I know this might sound manipulative, but I would bring up the subject several times with your daughter in a general context before coming out officially. My thought is that if this has never been a subject for discussion with her, allowing her to think about concepts like lesbian and gay in an objective sense before it all becomes personal may ease the way for her.
As the PPs said, she may already know more than you think, and bringing up the general subject of lesbian moms may prompt her to ask about your relationship on her own terms, without ever needing to have a formal Talk.
Best wishes to all of you!
Rhu - mother,grandmother,daughter,sister,friend-foster,adoptive,and biological;not necessarily in that order. Some of it's magic, some of it's tragic, but I had a good life all the way (Jimmy Buffet)
11 is a very sensitive age...try to get a feeling for where she's at in her head around people being gay or lesbian. It may help to know this and may help you determine if you can just be real honest and upfront or if you need to be real gentle and ease slowly into it. I wish I had more to say...
Goodluck. Let us know how it goes when you do tell her.
But just try to let her know that the love you have for her will never change and make it very clear that the relationship you have between your girlfriend and the relationship you have with her is not a competition that you will always love her