Question: Do girls and boys really need different toys especially when they are very young? I am speculating part of reason why girls tend to play with Barbie dolls and boys tend to play with transformers is:

* Parents make decisions for them.

Would girls play with Lego at all?

Answer: 
My wife thought the same thing until we had a boy.

While the range of enjoyment varies more within the sexes than across the sexes, in general, left to their own devices, girls will gravitate more towards traditional "girl" toys, and boys towards traditional "boy" toys.

Girls can (and do) enjoy lego, but they don't gain satisfaction from spending hours of time alone building towers and space ships. Boys will play with dolls (okay action figures), but not to the extent that girls will play with Barbies.

In our household, we base our decisions about what toys to buy based on what the kids actually play with when they get their hands on it. Most parents I know do pretty much the same thing. Most families have no time or energy to worry about the politically correct issue of the day.

My daughter was considered a tom boy. She often preferred playing with the boys to the girls, and had a selection of both "boy" and "girl" toys. Later, my son inherited many of her toys. He plays much more with the trucks and transformer action toys than she ever did, and not at all with the "girl" toys that still survive. 

Disney has not been able to break into the aftermarket for boys nearly the way they have with girls, and it's not for a lack of trying. Girls are eager to dress up as their favorite princess, but boys in general have no prolonged interest in dressing up as this or that pirate or prince.

In the mid 2000's Disney commissioned an anthropologist to study the culture of boys. What they discovered was that boys, as a whole, have an entirely different type of culture than girls, within the same macro-culture. [See Link](http://www.nytimes.com/2009/04/14/arts/television/14boys.html?pagewanted=all&mcubz=1)

Based on that research they had to invent a whole new marketing machine from scratch to reach the boys aftermarket. They went so far as too buy marvel comics and Lucasfilm (star wars). Yet they still have not been able to establish a solid aftermarket product that is particularly sought after by boys.
Ask any parent how many toys they've bought for their children that the kids outright ignored. There's a certain amount of cultural influence, but your kids like what they like. They have individual opinions and preferences from a very young age.

My eldest daughter has a very narrow interest in toys. She likes blinky lights and sounds, and will have one favorite toy she plays with until it wears out. People keep giving her dolls, which she has always completely ignored. We just sort of amassed a collection and kind of forgot about them.

Five years later, her little sister was born, and we had this big collection of unused dolls and other "girly" toys lying around in toy boxes, and she took to them like a duck to water. No one gave them to her, or encouraged her to play with them, she just found them on her own. Because of our older daughter we had sort of forgotten that little girls were supposed to like dolls.

So just treat your kids like individuals. They will definitely tell you what toys they like, and there's not much you can do other than introduce them to a wide variety.