Linda Taylor, 63, took her ten-year-old granddaughter to a pop-up Halloween shop in Bluewater, Kent, when she was horrified to discover that many of the kids' costumes featured bloody blades.
Among the costumes of witches and pumpkins for children were a collection of costumes sold with images of children holding weapons.
Linda, from Welling, Kent, said: "In today's world of knife crime, displaying photos of kids holding knives is completely inappropriate.
She said she spoke to a shop attendant and a security guard who said the ghoulish costumes were being sold.
Linda said: "These costumes are being sold nationwide. If you wouldn't want to see a child knifing someone, why would you want to see that as a costume?
"I work in child protection services. I know all about the horrors of knife crime.
"Can you imagine your son or daughter died from a stabbing and seeing a 5-year-old wearing that on Halloween?
"It's so horrendous. Why would you put that on a child? Why would you be promoting that?"
The costumes, on sale for three-year-olds and upwards, include a "psycho clown", "zombie clown" and "possessed doll".
Linda said she feared children picking up a knife at home to imitate the photo.
She said: "It really unnerved my granddaughter. Even she asked why a shop would sell costumes like that to children her age."
Halloween HQ, based in Ireland, occupies a retail space in Bluewater for the month of October each year, selling "costumes, make-up and accessories, home decorations, party and trick or treat related items".
A spokesperson for Bluewater said: "We cannot speak on behalf of an individual retailer, give information about their specific stock, or prohibit items being sold in-store that adhere to trading laws."
Halloween HQ have been contacted for comment.
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