@NBTX11, I would encourage you to start telling your children about polygyny as soon as possible if you think you will be living it at some point. They will need time to discuss things with you and to let their brains catch up to this very strange idea their parent is telling them about. It takes time! Sometimes years to truly come to terms with how to react to a parent who has turned into something different than what they were raised with and counted on all their life.
Also, you will be fighting in the Spirit realm at the same time. This is a VERY spiritual matter. Satan does not want it to move forward in anyone, especially young minds who can be swayed by culture. Your daughters will probably have more issue with it due to the Patriarchy that is foundational to plural marriage. I believe you to be a Patriarch in your family but you will probably be surprised with how much culture is in your daughter's minds, being influenced in the Spirit realm. This will take lots of prayer closet time to fight.
I say all this to say, talk NOW, not when you have a new wife to attend to and a first wife who will need assurances during the adjustment time at the beginning.