We have two Halloween parties each year - one for the children, and another one for adults.
I love getting everything ready for the little ones. We put fake cobwebs up, black balloons and carved out pumpkins with torches inside them instead of candles.
Games vary from as others've suggested, to treasure hunts and hide and seek. At the end of the evening, we release a net full of shiny shapes - moons, stars (can be bought at most good stationers), which've been suspended from the ceiling until our Halloween cake's cut.
The kids never want to go home - it's brilliant!