I don’t think it’s a sin. Romans 14 gives us freedom here, as long as our conscience is clear.
Church Halloween nights seem like a smart move to me. The kids are going to want candy and costumes either way, and I’d rather that happen somewhere they’re surrounded by believers than out in the dark. My line is anything that glorifies the occult or death for its own sake (like this one). A six-year-old in a Disney princess costume is just a kid in a costume. If your conscience bothers you, skip it. But a bag of candy isn’t a spiritual compromise.