Don't ever answer a question like this without first ascertaining why she wants to know.
Something like this:
DW: Why do you think that Joseph Smith wrote the BofM?
Jfro: I'm not sure you really want to know the answer to that question. I doubt that you trust my opinion so why would I trust you to really be an objective listener? It would be a waste of both our time.
But I want you to trust me so why don't you tell me why the BofM is so important to you?"
Then you need to listen. And when she's done, even consider not making much of a reply. Just say something to the effect of, "I understand. I see why you feel this way. I just want you to know that I love you and hope someday you'll have the capacity to understand me as well."
As Stephen Covey says: "Most people do not listen with the intent to understand; they listen with the intent to reply."
This is why she is asking the question. She doesn't want to understand you or your heart. She only wants to shoot you down.
But the way to overcome her distrust is to show that you do understand her fear and frustration.
Fear and frustration translate into anger. She needs to know that Heavenly Father is much more loving, kind and forgiving than she has made him in her heart.
Instead of making this about your disaffection from the church, change the discussion and make it about understanding her fears. Don't worry quite so much about your child. Kids are amazingly flexible and have a great ability to repair. At some point they will come to ask questions and you will have the answers. And who knows maybe Mom will be right there with you because she's grown out of the church.