"Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils." (1 Timothy 4:1)
Halloween was a corruption of "Hallowed E'en," the evening before "All Saints Day" in which civil disobedience and sinful license were tolerated prior to the forgiveness and penance sought the next day.
Although the level of debauchery and wickedness has waxed and waned over the centuries, nothing "hallowed" has ever been associated with the practice--until more recent times among evangelical churches. Now we promote a "Harvest Festival" or a "Bible Character Dress-up Night"--much of which encourages the practice of costuming and treats as a harmless alternative.
The difficulty is not with the church activities, but with the timing and the association with that which is evil. That obvious connection with a pagan holiday will undermine resolve to "come out from among them, and be ye separate" (2 Corinthians 6:17).
Yes, no doubt that passage warns against an "unequal yoke" in marriage--but its primary focus is on church and individual purity! "What fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?" (2 Corinthians 6:14-15).
As a parent, I know the pain of restricting my children from participating in the "fun" of Halloween. And as a former pastor, I know the pressure to accommodate the majority of church members who see no "harm" in such things. However, our allegiance and our responsibility are to the Lord, not men (Colossians 3:23). HMM III