I don't have kids and don't plan to, but my guy has 3 kids with his wife. They just knew me as "Miss C" (we live in the South and all adult women are Miss Firstname to be polite). I went on some family outings with them, babysat, spent time at their home, and they knew I was one of the adults who was "responsible", but we just didn't discuss the actual facts of poly.
Then again, they are 12, 10, and 5 - and the 12 year old has developmental/social issues that impact his understanding of relationships.
So .. maybe if we had stayed together as the kids got older and developed more of an understanding or had questions, it would have been more of an issue. For us it generally wasn't.