I do not pooh-pooh witchcraft and paganism and all the rest of it. I know people who have made those choices. I reject those paths. We serve Christ alone, who delivers us from the darkness of all that.
That said, when we do Hallowe'en (or Trick or Treat), we're not engaging in some sort of devil worship or social rebellion or anything of the sort. We're having FUN.
Someone once said that a Puritan is someone who is haunted by the idea that someone, somewhere, is having fun. And as Sir Toby Belch said to the Puritan Malvolio, "Dost thou think, because thou art virtuous, there shall be no more cakes and ale?"* Not that Sir Toby is much of a model for Christians, but still, there's nothing wrong with a little cakes and ale now and then. We ought to have room for fun.
Now, neither debauchery nor fellowship with devils is proper fun for Christians. But making fun of the devil IS. C.S. Lewis headed The Screwtape Letters with a couple of relevant quotes:
"The best way to drive out the devil, if he will not yield to texts of Scripture, is to jeer and flout him, for he cannot bear scorn." -- Luther
"The devil . . the prowde spirite . . cannot endure to be mocked." -- Thomas More
We need to point out that dressing up like goblins or monsters or witches is not about emulating such things, but making fun of them. For they have NO power over the Christian. Christ protects us. We don't need to live in fear.
For I am convinced that neither death, nor life, nor angels, nor rulers, nor things present, nor things to come, nor powers, nor height, nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord.
For that matter, I don't think trying to scare people out of hell/into heaven is much of an evangelistic technique. And I think some of those earnest Puritans trying to make the most terrifying presentation they can for their Hell House -- in order to assist the Holy Spirit's work in convicting kids of sin and moving them toward relying on Christ, you understand -- are secretly just enjoying all the gore. They get to wallow in all the forbidden stuff and feel virtuous at the same time. Which for some people sure beats cakes and ale.
*Modern paraphrase: "Just 'cause you got religion, you think that's the end of beer and doughnuts?"