Well, yes and no. It's a gimmick, and when your show already has vampires, or takes place on another planet, how are you going to fit in children and masks and make it interesting?
A variety of ways, it turns out. I'm just going to run through a few of my favorites here.
Probably the most straightforward example of this trope that I'm going to discuss is from Buffy the Vampire Slayer. Here we've got quite a few episodes that could qualify as implicitly Halloween-themed, but the two obvious ones are “Halloween” and “Fear, Itself.”
The twist in these episodes is that, in a world of demons and vampires, the baddies stay in on Halloween. Performing evil deeds when everyone around you is dressed up all scary in order to score candy? It's cheap. And so, Halloween is meant to be a calm and relaxing night—except, of course, unless you're Buffy or one of the gang.
While not exactly scary, “Halloween” is definitely an entertaining episode with several character twists—Xander as the capable soldier, Buffy as the maiden in distress, Willow as a sexy ghost. Whatever costumes the characters put on, they become. While the plot could be more gripping, the episode is very much worth it just for the clothing and the symbolism inherent in every character change.
Then we have “Fear, Itself.” As one might guess from the title, the fear factor for this episode is greatly improved. Here, we have a college haunted house that actually appears to be haunted. The gang is split up, characters are abandoned or attacked, and a demon has been summoned. With traumatized co-eds hiding amidst the cobwebs, and characters all alone in the darkness, the episode becomes a chillingly effective success.
I'll let you, reader, find out what happens to the demon. Let's just say he goes out with a bang.
Our next show is Castle. (While it's a bit of a stretch to describe this show as fantastical, these episodes either refer to Nathan Fillion's stint on the cult sci-fi hit Firefly or toy with the supernatural. They're also really fun.) Take season two's “Vampire Weekend,” otherwise known as vampires, goths, and ghouls, oh my! (Okay, maybe not ghouls.) This episode begins with Castle dressed as Mal, captain of the spaceship Firefly.
“Didn't you wear that, like, five years ago?” his daughter asks him.
“So?” he replies.
“So, don't you think you should move on?”
“I like it!”
The episode continues in standard fashion: a body has been found in a graveyard, a stake driven through the victim's heart. Castle and Beckett find themselves in a subculture of black clothing and synthetic fangs, and the possibility of real vampires excites Castle in a delightfully childlike manner.
Eventually, they solve the crime, and there is no immortal life involved. The journey there, however, is endlessly entertaining. Also, for those among us with an appreciation for the more Gothic side of the clothing spectrum, highly aesthetically pleasing.
More recently is the episode “Demons,” wherein Castle and Beckett investigate a murder in what appears to be a real-life haunted house, home to several murders. There are no Firefly references in this episode, but the was-or-wasn't-it-a-ghost tension works, and the banter is top-notch. Is it the best episode of Castle ever? Probably not. But it's certainly worth the price of the candy corn you'll snack on while watching.
Perhaps my favorite example of this trope, though also the least straightforward, is the Farscape episode “Kansas.”
John Crichton is an astronaut, trapped for 3 years in a distant part of the galaxy, who finally makes it home to Earth. Problem: he makes it back several years too early, in 1986. Problem: his friends are aliens, very non-human looking aliens, though mainly with two arms and legs. Sometimes with bonus tentacles or blue skin, but still, the general shape is the same.
Solution? Visit during Halloween. Also, focus your plot on John Crichton's attempts to fix the timeline and John's familial relationships. Just make sure to have several sugar-related jokes, and have a cop investigate our jolly group for their suspiciously realistic masks and limited English skills.
Truth be told, this episode is fantastic. By far the best I've discussed here.
John's homecoming to Earth does not disappoint, and the difficult relations between John's parents (with one another, with a teenage John, and with our adult John) pull at the heartstrings. Even Chiana's short fling with a teenage John does not disappoint, or disturb too badly. Perhaps the one downside is that this episode comes late in the show and is thus the culmination of a lot of emotional drama. After all, who would want to start watching The X-Files on the day Mulder learns the Truth?
In other words, Spoilerphobes beware. Everyone else? Please choose this scare.
Obviously this list is by no means exhaustive. While not strictly supernatural, Psych's “This Episode Sucks” is as silly as it sounds, and when Dead Like Me features children and costumes, it's a bit funny, a bit sad, and a bit horrifying.
These are some of my favorites, though, and if you're as in love with TV and Halloween as I am, they are sure to make for one awesome weekend.