Oh my! If I were just starting a relationship and the woman (or the man in my single days) began asking about children and all from day 1, I'd run the other way! It's nice that you want to cover all the bases up front, but really premature. Slow down, take a breath, and let things begin to develop naturally. If you are always thinking 10 years in the future you have no focus for the now. Relax and go at a natural pace for the three of you.
For poly to work well all parties have to be willing to keep communication open. If your wife is unable, it is something that should be worked on before adding another to your lives. While you may be able to handle it because you know her well enough to do so, once there are more people in the mix there are more opportunities for misunderstandings. Nip it in the bud.