And to play out DayTripper's always astute reasoning a bit further, if you tell them, will you be asking them to keep a secret for you? I think you recently posted on another thread that you had only told Bluebird (but my memory after reading a zillion posts is sometimes off).
I would at least go out of my way to expand their education to more than there's only "Dick and Jane" or "Barbie & Ken" in the world. If I recall there are some "kids books" in the gay/lesbian world that explain alternative lifestyle relationships in an accepting way so kids can accept gay/lesbian parents or family as "normal." You might visit B&N.com or someplace for some of these and see if you can work them into your daughters reading list.