You need to really look at what they want to know. Just like when any child asks "where did I come from?" It's like the 3rd grader who came home from school and asked "Mom, where did I come from?" The mother and father sat down and had this long discussion about eggs and sperm and pregnancy and delivery. When they finished, they asked, "Do you understand?" "Not really, Benny came from Baltimore. So where do I come from?"
We all tend to take their questions at face value, without asking them what they want to know and why. We'll keep on telling them that it took a lot of people to help mommy and daddy. Perhaps some day she'll ask if one of those people was an egg donor. Then I'll ask her why she wants to know and if it would mean that her mommy doesn't love her. I won't lie to her. I don't look at this as keeping a secret, only keeping private the information which is private to our family.
I've had a couple of people ask if we've used donor eggs, and I simply responded, "Why would you ask someone such an obviously personal question?" Most would answer, because I was curious, to which I reply that being over curious about others personal matters isn't an attractive trait. Those who ask because they are considering DE themselves, I answer that they should get in touch with an IVF clinic and discuss the matter with one of their physicians to see if it's what they need, or to look on the internet for some support site. My DH and I haven't discussed this with anyone outside of ourselves, and we won't. Also, prior to all this, we decided that in the unlikely event that we ever divorced, we'd not bring this up as an issue either and we made a written contract to that fact and it's sitting in our attorney's office with a copy in our individual safe deposit boxes also. Sadly, some families have met with divorce and suddenly this has been brought up by some husbands as a reason why they should have custody (fortunately, the judges have been smart enough to quell that right away). Fighting after divorce is bad enough for children, but fighting about their genetic origins is lower than low.