I am not sure how much help I can be other than to echo what others have said...
Mono people who express concern for the welfare of the children are most often concerned about some other issue entirely. I am blessed in a way not to have any family other than this fun and fascinating poly tribe that I am building for myself. However, when I have addressed people outside our tribe about the impact of our lovestyle on the children, I point out that the people I add to our family (even though they are my lovers) function as extended family to my kids. I never invite into my world people who are not willing to commit to my children even more greatly than they will commit to me. I also think that it is not much different than allowing Uncles to spend time with my girls. I choose people who can love and support my children and can also support my parenting choices. We are very open with the kids and they have been hugely excited as our family grows. We also have encouraged our girls to form close ties to people that are NOT lovers...friends, members of the Pagan community, etc. My children think of all of these people as their family and not potential threats to their security or to the core of their family unit. Very recently, my husband's brother began divorce precedings and is leaving his wife of 20 years. My children are saddened about the divorce, but their poly experience has taught them that they will not be loosing their Aunt. Bottom line...kids love and respond to those people who love and respect them. If you form an appropriate and close relationship with the child involved, there should be no reason for concern.