I'm not too hung up on the whole "Should Christians let their kids wear costumes and go trick-or-treating?" discussion. I'm sure true pagans scoff at our Americanization of their holiday and would claim that there's no real connection with Samhain or whatever else defines old-school Halloween.
However, there are two facts I'd like to point out on Halloween. One is that October 31st is also the anniversary of Martin Luther nailing his 95 theses on the door of the Wittenberg Cathedral, an event widely recognized as the beginning of the Protestant Reformation and all it stood for: salvation in Christ alone on the basis of faith alone in the message of Scripture alone.
The other point is this: we do live in a world largely controlled by evil spiritual forces: "For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms" (Eph. 6:12). "The powers of this dark world" is better translated as "world-rulers of this darkness," in the sense of "world-rulers over this darkness." Most commentators believe that such powers affect leaders of worldly governments. Christians, however, need not fear such evil IF they "put on the full armor of God" and "be strengthened in the Lord and in his mighty power" (Eph. 6:11, 10). The armor of God is God's armor given to the believer which allows him or her to stand against the "wiles of the devil," that is, the various schemes the devil works to battle against the kingdom of God (Eph. 6:11).
Now, a subpoint to the point above. Spiritual warfare is nearly always defined as something like "taking the battle to the enemy" or "storming the gates of Hell," using the sword of the Spirit (the Word of God) as a justification for making it an offensive, not defensive, endeavor. I think this is incorrect, and I think Ephesians 6 is on my side here. The word for "take your stand against" and the word for "stand" in Ephesians 6:11 and 13 are the same word; "stand your ground" in verse 13 is the same word but has the prefix "anti" attached to it. Primarily these words mean "to stand firm" or "hold one's ground in battle" (BAGD). Harold Hoehner, in his landmark commentary, states that "stand" in this passages means that the church is "only 'to stand,' hold the territory that Christ and his body, the church, have conquered."
Further, "the gates of hell" is a bad translation of pulai haidou. "The gates of Hades" is better, and we need to remember that Hades is the place of the dead, not necessarily the same place as Hell (gehenna in Greek). So when Jesus says, "upon this rock I will build my church, and the gates of Hades will not prevail against it," I believe the "rock" referred to here is not Peter (Petros) but the confession that Jesus is the Christ, the Son of the living God (Mt. 16:16). The word "rock" in verse 18 is petra, a feminine form of petros, which means that Peter is not the rock upon which Jesus will build his church. Though the world "confession" (homologia) does not appear, it agrees in number and gender with the word petra, and it is what Peter just did two verses earlier. What this means is this: the church Jesus has built will not be defeated by death because it is built upon a confession of the one who has conquered death, not because it is built on Peter (sorry, Roman Catholics).
All of this to say that Jesus holds the keys to death (the fact of death) and Hades (the place of the dead) (Rev. 1:18). Therefore, when people in the church (and remember the church is made up of people, not buildings!) die, the church is not defeated because the death of the believer is simply the event that ushers them into the presence of the resurrected, death-conquering Lord Jesus! One day, death and Hades (as well as all who once occupied them) themselves will be thrown into the lake of fire (Rev. 20:14, also another clue that Hades and Hell are two different places). The devil and his angels (yes, angels can be evil - Satan is a fallen angel) will one day be thrown into the lake of fire where they will be "tormented day and night for ever and ever" (Rev. 20:10).
The devil is not the King of Hell; he is its chief prisoner! He does not torture people in Hell; he is tortured for ever and ever. He does not enjoy Hell; Hell is not the Devil's paradise; it is his punishment!
Don't be afraid of Halloween. Know the facts: 1. Yes, spiritual forces of evil rule this world. But, 2. All evil spiritual forces will one day be thrown into hell to be tormented for ever.