"Girls want superheroes, AND the boys want superheroes!" a little girl from New York knows what she's talking about.
Children are being taught gender roles everyday, by parents and toy companies alike. If you give a three year old girl a science toy for her birthday, her friends will exclaim, "That's a boy toy!" Children don't come up with these opinions on their own. Someone told these little girls they can't like "boy toys."
Why can't they? What atrocity will happen if she picks out Spider-Man instead of the baby doll?
Or if he plays in the toy kitchen?
Mike Reynolds, from The Good Men Project, was tired of being asked why he didn't feel the need to have a son (having already had two girls.) He wrote "The List of Things I Can’t Do Because I only Have Daughters." It is a sarcastic and wonderful article poking fun at gender roles.
A Mighty Girl is a site and Facebook page focused on girl empowerment. They have a lot of resources for parents that want to break from the "norm" and give their girls educational and inclusive toys, books, and costumes. Do a search for "superhero" on the site.
There are plenty adult female fans of comics, video games, and D&D, but why is our culture trying to stamp that out for the next generation?
Comic books do have issues with how they depict female heroes/characters. Some artists draw very exaggerated and sexualized poses and anatomy. Check out The Hawkeye Initiative. The site is a comical look at this problem. They take real comic book pages featuring female characters, and put Marvel Comics' (male) character Hawkeye in the same poses. The result is hilarious, and sometimes unsettling. If artists were more concerned with how women and girls would react to this (if their daughters were reading it) maybe they'd think for a moment about how they do this unnecessarily. Obviously there are graphic novels geared towards adult readers, but this happens in scenes that should (normally) be acceptable for younger readers.
Bronies are challenging fandom gender roles. Check out Bronies: The Extremely Unexpected Adult Fans of My Little Pony for information on the movement. (Here is the trailer.) Many of these fans of the animated ponies are grown men, but would you let your little boy watch the show? Or buy the toys?