Put appropriate children's Gospel tracts into baggies with candy to give out on Halloween. Include an invitation to Good News Club, Wednesday night clubs or church and sunday school. Encourage the children in your club/class to do the same.
Go a step further by teaching a party club using a lesson such as "Are You Afraid" (available from Child Evangelism Fellowship) to communicate the Gospel to the children (and maybe the parents too, who should always be invited.)