I've helped with my child's school Halloween parties for the past 3 years (grades K-2). The favorite games for that set that I've seen:
1) Blow up orange balloons and put jackolantern faces on them. Give each kid a balloon. They all have to hit their balloon into the air, and whoever keeps it up without hitting the ground the longest wins. To change it up, have multiple rounds. In round 1, you hit the balloon any way you want. In round 2, you can only hit the balloon with your head. In round 3, you can only hit the balloon with your elbow. Round 4, back of your hand, not the palm. (TIP: Do NOT have a round where they use their feet or knees, someone WILL get kicked, not good)
3) Carve a pumpkin so the mouth opening is flat to the ground. Get a golf putter (cheap plastic outdoor sets for toddlers are perfect) and have each child take a turn trying to putt the ball into the pumpkin's mouth.
4) Make round sugar cookies, and bring a batch of orange icing, and some tubes of black icing. Let the kids ice their own cookies, and draw a jackolantern face on the cookie.
5) All the kids sit in a circle and play hot potato, but instead of a potato, pass a small stuffed pumpkin (or even a balloon from game 1).
6) Bring paper bags (little treat bags) and a bunch of halloween stickers. Let the kids decorate treat bags. They can use these to collect their prizes from the games, and for a treat to take home.
7) Freeze dance to halloween music.
Get a bag of halloween candy, and for each game, pass out candy prizes to the winners. At the end, divide up the remaining candy among all the kids so everyone goes home with some candy in their bag, even if they didn't win a game.