If you have to say what your costume is, it needs work. These characters don't need to say that.
"Seems like Halloween in this world is the same!"
Holidays are almost always the same in fantasy manga. Sometimes they have different names, but that's about it.
"Since nobles aren't supposed to interact with commoner kids, the ones who actually dressed up and handed out the sweets were the mansions servants."
Dressing up in costumes is a perfect excuse to actually interact with them, though.
Those costumes are very bland and stereotypical. Nothing like what they hype them up as.
Dullahan is pretty hype, though.
Stones in candy is generally not a good idea, unless it's very obvious.
That's not a common face for her.
So how did the helmet get there as well? Wasn't the window inside it?