In our main poly relationship years ago, our significant other had 3 kids, and we had one. She often babysat for us, and I walked her youngest to school every morning. I was there to meet the kids when they got home from school, and we watched TV or played games together. I read stories to all 4 of them every night. It was one big happy family, and I have to concur with the others that more adults makes for a better environment for the kids. There is more support, more love, etc. When it came to decisions, we made judgment calls about our one, and she made the decisions for her three. It ran pretty smoothly. Her kids' father was often over and we got along great with him. All in all, it was very harmonious. Too bad it didn't last forever.
Today, we still have a few honorary aunts & uncles. The kids know which aunts/uncles are related to them, and which are not. Some are here in town with us, and get together for holidays, while others just call on the phone or send gifts & cards to the kids. To them, it is really no different than having regular aunts & uncles. They know that some are Mommy's friends or Daddy's friends, or both.