In kindergarten, I think it works best to have children divide into groups that travel to different centers. Have about 4 stations that kids rotate to every 15 minutes. In kindergarten, I played Halloween bingo with approx. 5 kids at a time. I had little trinkets to give the when they won. I printed free customized Halloween cards from dltk.com. (A great site especially since you can customize charts.) Another mom had a station where the kids decorated foam photo frames with foam stickers. The class took a group photo right when the party started. A dad drove to Walgreens and got 20 copies made of the photo and returned back in an hour to stick the photos in the frames in time to take home that day! One mom was in charge of a game similar to Hot Potato but she used a pumpkin. At another station, kids decorated a Halloween cupcake. There are tons and tons of ideas on http://familyfun.go.com/halloween/
Have fun! Our PTO gives the room mom $2. per child for each party that our school has. (We have two parties per year.) Oh... we did get all the kids together to play one large group game. We did relay races in the hallway.