Depending on the costume, sometimes it can be made scarier by actions.
My husband dresses up in full pirate regalia for Halloween, a costume he put together over the years from various Renaissance faires. We only have one Trick or Treat event, on Sunday afternoons around the holiday, so we get all ages throughout the event.
When the older kids come up to the house, he comes out brandishing his sword, & menacing, they love it!!
But for the little ones, he walks up to them and asks if they've seen his parrot flying around. And lets them touch his cloak, etc. so they aren't scared.
If you happen to be near a small/young child who seems very unsure of your costume, find a way to "normalize" it for them, either taking off the mask as you approach them & asking if they want to see it before you put it back on, so they know you are a person & not a monster, or finding a "lighter side" of the character to engage in with them.
Happy Halloween! T.