Don't tell her. Why? What would be the point? You will only confuse her and hurt her. Why buy into the borg's ideology that if you are leaving/if you have committed a 'sin'/if you have do ANYTHING agaisnt what they preach, that you must CONFESS, TELL ALL. Your spirituality is between you, your husband and God. It is no one elses business what you do with it. If she ASKS someday, then that is different, you could brush it off with, "Oh that, well your father and I are doing some serious thinking." I personally would never let the organization and their stupid rules jeopordize my relationship with my daughters. Don't let them! Just live your life and when it comes to religion, skirt the issue. Or feed her with information very very very subtly after the wedding day.
My regret is that I did not keep my mouth shut more when I was not df'ed. I thought the elders and everyone had a right to know what was going on in my life. They don't. I wish I had never bought into that whole idea. Their true nasty colors come out when you are opposite to what they think you should be, not their love and understanding. Sadly, your daughter is lumped in with them and will feel she has no choice but limit contact. AND think of grandchildren someday. DO NOT ALIENATE yourself!!!!!