What's the utility of this revelation? Consider for a moment that your child called you and said they've considered the evidence and are now firmly in the flat-earther camp. How does that change day-to-day actions or relationships? It really doesn't. They may be insufferable when conversation circles around to the moon landing or air travel, but that can be handled by placing firm boundaries. "I disagree with you and you know where I stand but I value our relationship enough not to bring it up again. I ask the same courtesy from you."
If your child uses this revelation as an excuse not to engage in plural marriage then you'll find that you agree more than you disagree. If your child uses their revelation on polygamy as a lens to view the Restoration, I would hope they could be like David Whitmer - who had significant differences of opinion on Joseph's revelations and how the Church should be run but held fast to his testimony of The Book of Mormon. Similarly I would hope your child would hold fast to that testimony which they've already gained. Once again, if this is the path chosen, you'll have quite a bit in common and can continue to share that.
Finally, it's possible that your child is taking a big step to leave the Church of Jesus Christ. Just so you know, 2/3 of those who leave the Church leave religion altogether. Even if this does happen, there are still some values that your child will keep. You can focus on those values you continue to have in common.