Kidscape said the trend objectifies youngsters who have no say in how ridiculous (or cute) they look. It warned that children are being treated merely as party accessories.
Earlier this month, Peaches Geldof posted pictures of her five-month-old son Astala on Twitter, dressed as a bear and as a dinosaur. And in the past, singer Dannii Minogue has shared pictures of her baby son Ethan, dressed as a pumpkin.
Costume shops have reported a surge in sales of pre-made baby costumes, with high street stores, including Marks & Spencer, Asda, Toys R Us and TK Maxx, all now offering a range of fancy dress for children under 12 months.
One costume retailer Emma Angel, director of Angels Fancy Dress in London, told the Daily Mail she had seen a 'significant' rise in sales of baby costumes this year:
"There is undoubtedly a growing trend regarding fancy dress costumes for babies, and sales of such products have increased significantly at Angels this year.
"Noticeably, the most popular costumes are animal-themed, and this Halloween our best-selling products for this age group include baby bats, dinosaurs and spiders.
"In fact, demand has far outstripped our expectations and we have had to order in more stock."
Other British retailers said they took inspiration from America, where baby costumes include outfits designed to make them look like hot dogs.
However, Kidscape director Claude Knights said: "Many parents will just see this as a bit of fun. But we should maybe think a bit harder about what message we are sending out about our attitudes to children.
"There is a growing trend to accessorise babies. Some parents make them fit in with their own style, rather than treating them as individuals.
"It is not like older children dressing up for fun. It is potentially objectifying babies who have no say in what they wear.
"Obviously it is not directly harmful for the baby itself. But it says something about the attitude of the adults who are meant to nurture that infant."
She added that Halloween risked commercialising children in retailers' quest for profit.
"Halloween used to be much bigger in America than in Britain," she said. "Now hard-pressed families over here are spending more and more of their cash on Halloween accessories and costumes.
"It is a great marketing opportunity for businesses. As the market in teenage and children's products becomes saturated, they are turning their attention to young babies.
"It is all part of the creeping commercialisation of childhood. Yes, it can be fun. But what does it say about our society?"
What do you think about this? We rather like a little cutie in a pumpkin. Take a look at these snaps if you need some convincing...