First, check-out
PolyMatchmaker.com.
How does having a poly relationship effect children? Well, from our own experience it's simply Uncle whoever (Our one real poly relationship was a boyfriend of Mrs. WS). Our kids accepted it without question. If we felt he was okay, so did they. And he did take very good care of my family when I was traveling and it was a goddess-send to have him there.
We know several other poly couples who have had live-in couples and singles and they don't over-explain anything to their kids. They give them the basics that this person (people) will be living with them and that's it. If they ask more they tell more. But they don't offer more information than they have to.
Mr. WS