From the linked article:
Paul said in verse 25, "Eat anything that is sold in the meat market, without asking questions for conscience' sake." This is most interesting. He says it is okay to eat the meat bought in the market place even though that meat may have been sacrificed to idols.
Then in verses 28-29 he says, "But if anyone should say to you, 'This is meat sacrificed to idols,' do not eat it, for the sake of the one who informed you, and for conscience' sake; 29 I mean not your own conscience, but the other man's; for why is my freedom judged by another's conscience?" (NASB). Paul is saying that if you find out the meat was sacrificed to idols, don't eat it -- not because of you, but because of the other person. In other words, eating that meat won't affect you. But, it may affect the attitude of another who does not understand the freedom the Christian has in Christ.
Is it any different with Halloween (or Christmas)? No. Even though Halloween has pagan origins, because of your freedom in Christ, you and/or you kids can dress up in costumes and go door-to-door and just have fun. However, if you are not comfortable with doing this, then you should not. If you know of a person who would be stumbled by doing it, then you shouldn't either.
The author does a pretty good job of explaining what Paul was talking about when it came to EATING MEAT sacrificed to idols. However, he errs in extending this principle out to Halloween.
Paul doesn't say that idol worship, idol sacrifice or any of the practices associated with paganism or idoloatry are acceptable. He merely is saying that eating meat that MAY have been used in an pagan sacrifice and later sold in the market is a matter of conscience.
A more apt analogy would be to say that if a Christian is offended by EATING CANDY collected by trick or tricking, then they shouldn't eat it.
Note that Paul did NOT say that we can go ahead and participate in pagan ceremonies. If Paul were writing about Halloween, he wouldn't say that we have "freedom" to participate in these pagan ceremonies. He would say that we should use our best judgement if we bought and consumed candy used in this pagan ceremony.