I have been to a few for mama friends and they differ so much with each mama! At one friend's there were kiddos everywhere, and the mood was still mellow! The lights were dimmed and candles were lit (and placed up high), there was mellow music on and it was in the evening. I think the mood of the adults really focused the energy of the children. Also, the children were part of the activities! The Mama-to-Be sat in the most comfy chair and the kids (and the rest of us) brushed her hair, rubbed oils onto her arms and legs, massaged her gently wherever she felt comfortable, and fed her fruits and such. It was so over the top, we all had a ball! The kids also helped with belly casting and presenting gifts (small gifts or homemade gifts were allowed at this one).
At others we've done belly casting, bead making (with Sculpey I think), stories/poems/songs... one friend passed out a candle to each guest for the candle/labor chain (I don't remember what it is called, but it is where one person is called when labor begins and then they call someone else to tell them, and they light their candle and call the next person... the mama is supposed to give birth before all the candles go out). One mama had people bring a greeting for the new babe and put them all into a book. I think she had paper and pen/markers set out to make something if you didn't bring one.
In Pam England's Birthing From Within, she says that it is important to eat "savory, delectable edibles, foods that symbolize strength in labor and other energies or qualities you will need to birth." She also suggests making a Birth Bundle, and sharing it in order for friends and family to bear witness to these symbols and insights within it. It should include three objects: A symbol of you, the Mother, a symbol of the energy of the Father or your birth companions, and a symbol to "remind you that you and your baby are working as one to be born." I really liked this idea, but never did it before DS was born... maybe this time around!