I tend to be flexible on these things. There were many parties where I had to take DD2 as she was feeding, just plain clingy or DH was away and I had no choice. Except for when she was a tiny baby I would always ask and explain the situation.

With holding parties myself, we always say to other parents that if they need to bring along a sibling that it is ok and actually is encouraged. At the kids ages it just seems appropriate for now.