My situation is far different as my primary relationship is unable to bear more children. So in our case we would be very happy to have children from a secondary relationship (I.E. a secondary and myself) as long as they were also comfortable with the situation.
I think children and poly all around is a very touchy situation. From what I've read many poly people are past childbearing age when they branch out into the poly world, and others simply never were interested in children. When you get the combination of childbearing age poly and interest in children you branch into a whole new realm. Then you're looking at the complicated dynamics of multi-parent households (more than two) is everyone on equal footing as parents to the resulting child(ren), who has legal say over the child(ren), how will the birth certificate read? What happens if the relationship does not last long term, does the non-birth certificate parent (in your case if your husband had a child with her, you) get any time or say in the child's life? It is a very touchy situation and thus one I think many people try to shy away from just due to the legal and emotional nature of possible entanglements.
Me: 30ish bi Female S: mid 20s bi MtF transsexual (presurgery)
Our Kids: D 8 yrs, boy (Mine only from prior relationship); T 2 yrs, boy; A under 1, boy
Living and Developing a relationship with L: 30ish bi Female and her husband B: mid 20s bi Male
Their Kids: Little L 3 yrs, boy, R nearly 2yrs, girl, Due November 2012
Favorite Poly thought (if you recognize help me find the source): Jealousy is not a disease, it is a symptom. To "cure" it you must identify the disease.