(CNN) I have no idea why my 3-year-old wants to be a cowboy for Halloween this year, because she won't tell me. That's OK. I don't need to know her reasoning to make it happen.

Thanks to her outdoorsy grandparents who live in the Colorado mountains, we already have the cowboy hat and handkerchief. We found a great pair of overalls on sale for $1.69 at the local Salvation Army store. But we still needed a cowboy jacket, a flannel shirt and boots. And whatever else might make her look like a 3-foot version of a cowboy.

Entering a temporary Halloween store that popped up in our town, I headed to the kids' costume section. The little girls' costumes seemed odd to me -- so many little skirts attached to superheroes and vampires. Even if I could find a cowgirl costume among all the girly-girl stuff, my girl would probably find a cowgirl skirt pointless. How would she ride her toy horse with a skirt? Sidesaddle?

As I turned toward to the boys' section to continue the search for accessories, I saw the costume that scared me more than any vampire or ghost: the little black dress.

Posed seductively like a brown-haired version of JonBenet Ramsey, the child model on the package looked 4 or 5 years old. I couldn't imagine the photographer asking that little girl to pose that way.

Americans are expected to spend $2.5 billion on children's costumes in 2015 , almost triple the $840 million they were expected to spend in 2010, according to a National Retail Federation survey. From my limited scan of Halloween costumes on sale this year, sexy costumes for younger girls are flooding the marketplace.

Many women I know have that little black dress, a somewhat slinky number that could be a cocktail dress or a basis for a suggestive black cat. But a grown woman makes a choice to dress in such a sexually suggestive outfit for Halloween or any other night of the year. She knows a sexy dress is going to garner a different reaction than Dorothy from the Wizard of Oz, but she can take precautions to stay safe from the weirdos. A child has no idea what's going on.

"The message to our girls (with these costumes) is that they can't be too sexy, and they can't be sexy too soon," says Melissa Wardy, founder of Pigtail Pals , a clothing line for adventurous girls, who has blogged about sexy Halloween costumes. "It's disgusting. She just potty trained. There's nothing sexual about her."

It can also be dangerous.

"Dressing girls like grown women for Halloween communicates that they have the sexuality of adults, in the bodies of children," says Teresa Downing-Matibag , an assistant professor of sociology at Iowa State University. "While little girls themselves likely have very little awareness of adult or even adolescent sexuality, or what sex is really about, the adults who are seeing them on the streets do. We are also communicating to adults that little girls are sexually appealing, and this message has tragic implications for their vulnerability to sexual abuse."

The sexualizing of Halloween costumes for young girls is part of a larger marketing trend emphasizing girls' outsides over their insides, says Michele Yulo, founder of the Princess Free Zone, featuring superhero Super Tool Lula.

"Marketers basically decide what is 'girl' and what is 'boy,' which ultimately leads to two very distinct ways to profit as well as a blind acceptance from the consumer who, often, doesn't realize it's happening," Yulo says. "Girls are inundated from a very young age with inappropriate messaging by way of products like 'skinny' jeans for toddlers, or T-shirts that say, 'I'm too pretty to do homework, so my brother does it for me,' and sexy Halloween costumes. Kids begin to codify other kids by placing them in strict categories that they've been taught -- not born with. Inevitably, kids begin to believe that girls need to be sexy, and boys believe it, too."

What girls hear is who they are is how they look, and how they have to look is "hot," which is creating problems for girls at an ever-younger age, says Peggy Orenstein , author of "Cinderella Ate My Daughter: Dispatches from the Front Lines of the New Girly-Girl Culture."

"Girls are learning at the youngest ages that their value comes from how they look, and the definition of attractive is very narrowly defined as being sexy or sassy (for the younger girls)," Orenstein says. "Sexuality is imposed upon them inappropriately, and they are encouraged to define their bodies -- not by how their bodies feel to themselves, but by how they look to others. This creates vulnerabilities for girls to the pitfalls we worry about -- distorted body image, eating disorders, depression and unhealthy sexual behavior."

Diane Murray of Wood-Ridge, New Jersey, has given up on store-bought costumes for her kids after ordering a SpongeBob SquarePants costume that turned out to be less than she had bargained for. Although it was supposedly the correct size for her 10-year-old daughter, "the shirt was tight and the skirt was miniscule, barely covering her rear end."

"I was thinking how embarrassed I would be to have her walk around in that costume, and she was embarrassed as well," Murray says. "Dressing up as a sexpot is not part of my vision for a happy, healthy childhood. Outfits like that not only suggest a style of dress that is inappropriate but also emphasize the notion of dressing to titillate men, rather than dressing in clothes that make you feel good about yourself."

What can a parent do this Halloween to protect kids from growing up too fast?

Emphasize your child's imagination. Yulo's daughter is going to be a baseball slugger -- no skirts please. How do you slide into home plate safely in a skirt? (Even though Geena Davis had to do it in "A League of Their Own.") Murray's daughter is going as a Goth girl -- all black clothes and makeup.

Other ideas: "Girls can dress up as dragons, monsters, spies, secret agents, mummies, queens, and even Peter Pan and the Fairy Godmother," Downing-Matibag said. "They can be Captain Hook, Santa Claus, the Lone Ranger and cowgirls, as well as lion tamers, scary clowns, astronauts, firefighters, sea captains, rock stars and Indy 500 racers."

Think comfort and safety. If your daughter will be running from one house to the next, make sure she wears shoes she can run in and warm clothing that protects her from the cold. If she's bundled up, she's not likely to be wearing that little black dress.

Think cheap. My neighborhood listserv is filled with parent requests for costumes in specific sizes to borrow, rent or buy. I know someone is going to enjoy my daughter's infant ladybug costume this year. I received it a couple of years ago from another parent who had bought it first. There's also inspiration at your local thrift store, where I got part of my daughter's costume cheap.