Halowe'en parties tend to go over the top in the last few years, and of course you can add to the mood with costumes (how about a prize for the best one) and brighten up (or darken down?) the house with some scary decorations. Get a few masks in case not everyone has one.
The games you play will depend on how old the children are - it's not a good idea to do anything too scary with young children. Little ones may enjoy traditional Hallowe'en games like apple bobbing or snap apple, or Noises in the night (this may be a bit scary for very young children). You could also try winking murder as a group warm-up, and for more active entertainment try sardines or murder in the dark.
If you have a garden, it's not too cold and the children are older, you can have great outdoor games - any of the 'hide and seek' type games are more fun outdoors, especially if it's getting dark.