(CNN) Every year, kids are asked a simple question: "What would you like to be for Halloween?"

Young boys might be steered toward G.I. Joe, Han Solo, Robin Hood, a Ninja Turtle, or any character from a long list of superheroes and action films and TV shows. For girls, at least in the last decade, Halloween costumes largely gravitated toward princesses.

"I remember 10 years ago, [when] trying to find a costume for my oldest daughter, it was very hard to find an option that wasn't a princess," said Carrie Goldman, author of "Bullied: What Every Parent, Teacher, and Kid Needs to Know About Ending the Cycle of Fear."

"Girls didn't have the choice to be Chewbacca or a Power Ranger."

Indeed, for the past 11 years, princess costumes have been the top-selling children's Halloween costumes, according t o the National Retail Federation . But this year, kids' superhero costumes are now the bestsellers for the Halloween season, bumping princess costumes down to No. 2 on the list, the federation said. Animal costumes, Batman and Star Wars characters round out the top five most popular children's costumes on the federation's list this year.

Not only have the Halloween options for kids gotten more diverse, they've gotten more empowering.

Princess costumes: The good and bad

For kids, dressing a a superhero meant they could see themselves doing everything their caped adventurers could do: they can fly, crush cars, chase bad guys and shoot out spider webs. Dressing as a princess often meant being pretty, but not necessarily much more.

"If you look at any [costume] display, the message to children is clear: Boys should be superheroes -- brave, active, extrovert. And girls should be princesses -- pretty, quiet, restricted," wrote Megan Perryman, a spokesperson from Let Toys Be Toys , a UK campaign group supporting non-gendered toys, in a blog post.

"Princesses are often objects, just meant to look pretty and wait to be rescued," agreed Goldman, who has three daughters. "I think there has been a shift in how young girls see themselves. Now, they want to be out there having fun."

Since "having fun" in superhero terms means saving the world, those costumes are generally more fit for activities than princess attire. For example, the costume for Flash, a hero with super speed, includes an athletic red jumpsuit. Cinderella, on the other hand, comes with a puffy blue dress and plastic high heels.

"There's not much action happening with the little heels," said Christia Spears Brown, a psychology professor at the University of Kentucky who studies gender stereotypes in children.

Instead of playing a pretty princess sequestered to a castle, Brown's daughter is dressing as Batman this year. "When you're a princess, the focus is on your looks. As a superhero, the focus is on your powers," said Brown.

Girls who do choose to be princesses, Goldman says, are more likely to pick modern characters with more agency, compared to weak heroines of the past. Costumes from the 2013 Disney movie "Frozen," for example, were top-sellers. "For Anna and Elsa, [both princesses from "Frozen"], it was all about the action. In the end of the movie, Anna chooses her sister over a guy," Goldman said.

Brown calls princesses like Elsa and Anna an improvement on those of the past, because "they're not waiting on the boy to save them." Still, the mother of two wishes that princess movies didn't always involve a love story.

"When girls are this young, they don't care about boys -- they just want to go on adventures," Brown said. "For superheroes, love isn't in the storyline. They're out having cool adventures and saving mankind."

Girls are often pushed to be princesses at young ages, Goldman explained, because "kids will emulate what they watch" on film. It can be helpful to explain what being a princess means to your daughter -- and maybe encourage her to seek out other heroes, too.

Superhero costumes: The good and bad

When Brown's 4-year-old daughter, who is now 6, wanted to be a princess for Halloween, Brown asked what exactly her daughter liked about princesses.

Her daughter said she liked the sparkly outfits.

JUST WATCHED 6-year-old girl becomes a superhero Replay More Videos ... MUST WATCH 6-year-old girl becomes a superhero 01:31

"There's nothing wrong with sparkles," Brown said. "But Wonder Woman also wears sparkles, and she does stuff."

Brown convinced her daughter to be Wonder Woman that year, and she's been a superhero ever since. Recently, her daughter has ditched the girl superhero costumes for boy versions of Captain America and Batman, complete with inflatable muscles and plastic gadgets.

Her daughter "doesn't like the girl superheroes as much, because they can't do all the things the boy superheroes can do," Brown said. "Girls can't run and be active with short skirts and boots."

When flipping through a Halloween costume catalog, Brown's daughter will often critique the girls' costumes: "She can't fly with this skirt on!"

Short skirts, leotards and tights are common in girls' superhero costumes, a frustrating reminder that even superheroes can be sexualized and marketed toward girls. For example, Harley Quinn, a female super villain from the recent film "Suicide Squad," is popular among girls. But her costume -- short spandex shorts, a skin-tight top and fishnet tights -- seems sexy rather than powerful, Goldman says.

"I've seen ["Suicide Squad"] and liked it, but my daughters are too young to watch it," Goldman said. "It definitely makes me uncomfortable to see young girls wearing shirts with 'Daddy's Lil Monster' across the chest."

Instead, Goldman thinks a better role model is Supergirl from the eponymous CW show -- she wears long sleeves, a skirt, and doesn't show cleavage.

Parents may need to say "no" to a costume that's too sexy, Brown said. "It's helpful to explain why, for instance, a short skirt isn't the best alternative for a superhero. Superheroes want to be more athletic."

Nontraditional costumes are more fun

Age-old conventions around dolled-up girls and macho boys are still robust today, and they especially impact children, Goldman said. Letting girls dress as superheroes not only broadens their costume choices -- it may also lessen the stigma of being "un-girly."

"I think there will be more acceptance of boys and girls if costumes don't box them in," Goldman said.

Join the conversation See the latest news and share your comments with CNN Health on Facebook and Twitter.

Jess Day, another campaigner for Let Toys Be Toys, agreed.

"Costume and fantasy play are fun, but if kids are only ever offered certain options, then so much creative play is closed off to them," she said. "Children should feel free to choose [costumes] for themselves, without having to worry whether it's 'for boys' or 'for girls.' "

This year, Goldman's youngest daughter, who is 6, is playing a different type of hero: George Washington, from the hit Broadway musical "Hamilton," her daughter's favorite. After some online searching, Goldman was able to find a tiny costume of the first president.

"The General is her favorite character," Goldman said. "She can't wait to scream, 'Here comes the General!' on Halloween night."