In this situation, I'd tell my daughter I had a problem with my breasts and was going to the doctor to have it fixed. I would explain more in detail if she asked. There would be no need to bring up sex, because, hopefully, she is not aware of that yet. If my husband and I modeled an accepting attitude, emphasizing that he loved me the same before and after, I think that would confirm to my daughter that worth/lovability and physical appearance are separate things.
My daughter is only four now. But I know there will be a time when we will encounter issues with appearance, whether it's weight, acne, makeup, or something else. I plan to teach my daughter that loving yourself and doing nothing to change your body are not necessarily synonymous. The key is to not let self worth be dependent upon physical appearance. No one has a "perfect" body. But each person is amazingly, wonderfully made and infinitely important. I want my daughter to realize this and to be able to deal with physical problems with an attitude of acceptance, not esteeming herself lower because she has problems, but stronger because she's dealing with them. A woman's true value lies in her heart, mind, character, what she means to and does for others. A woman's body is merely a vehicle that carries her true self through this life. If the vehicle has a problem, we take care of it in order to enrich our own and other's lives. But the real treasure lies within.
I hope this helps! God bless.
"Favour [charm] is deceitful, and beauty is vain [empty, fleeting]: but a woman that feareth the LORD, she shall be praised." Proverbs 31:30 (KJV)
"God grant me the serenity
to accept the things I cannot change;
courage to change the things I can;
and wisdom to know the difference.
Living one day at a time;
Enjoying one moment at a time;
Accepting hardships as the pathway to peace;
Taking, as He did, this sinful world
as it is, not as I would have it;
Trusting that He will make all things right
if I surrender to His Will;
That I may be reasonably happy in this life
and supremely happy with Him
Forever in the next.
Amen."
--Reinhold Niebuhr