You start off by isolating them into "dollhouses" and "action-adventure" things, marketing them to boys and girls separately on television (boys during Saturday morning cartoons, girls during whenever bored moms are supposed to be watching TV). Pick fairly gender-neutral colors, so that the girl ones aren't all completely pink. Part of the draw is that it's supposed to be, well, kinda realistic, so sell something that looks like a normal house on the street and something that looks more Victorian or whatever. A couple different things you're advertising with a burger joint, something like that, a social venue that a boy would also see as integral to a town. The key is creating environments that girls and boys would both recognize as important to their fantasies, but both would use differently with stereotypical play.
I see an ad that starts with an adorable little girl in pink overalls with flowers, white shirt, and a slightly lighter yellow construction helmet than the usual "boy" model. She has lots of fun building and customizing. Then the next shot has hair barrettes instead of the construction helmet and she's doing stereotypical dollhouse things. But nowhere does it say "for girls"--it just says "Lego." Cute, bouncy music, girls push their houses together and suddenly they have a town and they get to have their friends visiting. Marketed as social. Traditional. Family values.
(I don't know why I start thinking in Rorschach's voice when I'm trying to think up marketing for little girl toys, but whatever works)
In the stores, we're going to have those sets just below the adult eye level one, or above, so they're easy to find, but not too easy. The adult eye level will have slightly more upscale sets, stuff that blends male and female things. A castle is like a super-dollhouse. Princesses need castles. Every girl is a princess. Every boy is a knight. It's perfect (an ad with girls and boys, the girl maybe in a princess-ish dress, the boy clothed in a way oh so slightly reminiscent of the military; the princess and her handmaiden is enjoying the wood, the knights are sparring, and then a little red-headed girl, slightly more attractive than the others, bursts in with a dragon, general calamity, unification against this front, dragon with forest sold separately, another set with horses, saddles, and a unicorn to kill or tame; it's a toy that children can play with together, and more significantly, it's "fun for the whole family," all of your children, more important than ever in this economy). You know what kind of scandals you could have going on with the queen and the knights? An Arthurian one, that's what. Toys with just girls aren't nearly as much fun as toys that also have boys to fall in love with you.
And on the bottom shelf, in the kids' eye view, we're going to have the stuff that you don't need cognition to appreciate or think about. Pirates (with a pirate queen, that's classic, it's royal), brightly colored stuff, whatever, and just below or above that, a bit more or less attractive, companion sets to the mainstays, things you reach for because oh mom it will go SO WELL with my castle, like more knights and ladies, or a set with a rugged hunter, another way to get that much-desired unicorn and some more pretty plants.
And all this you put out under the name of Classic Fun. Classic girls, classic boys, classic ideas, but the point is that we don't have two points but a spectrum which somehow fits together perfectly, dovetailing (the adventures of Milly Sue in space sold with a girl fighter pilot), once the little girl figures out that she can pop some heads and get some dashing female knights), and the kids who want to play differently will be able to do so easily--and as Lego is sold as a Smart Yuppie Kid Toy soon enough it will be girls as well making palatial structures, and fathers proudly saying My Little Engineer.
And from there, Tonka and other similar companies start taking the angle because hell, it's a good business idea, and suddenly we are no longer the only girl construction toy AND WE WILL FIGHT TO THE DEATH FOR OUR MARKET, THAT WAS OUR IDEA, YOU BASTARDS.
Some of this may seem a little bit odd or rushed, as I was interrupted in the middle, but I think it'd be easy. And more than that, I think it'd work.