At the first and second grade level I see little difference in subject matter choices between boys and girls. However as the child passes the age of eight, the traditional, expected, stereotypical choices begin to emerge. It is at this age that boys and girls are starting to explore their sexual roles. With the girls, all is cuteness, sweetness, hearts and flowers (I know it's not P.C., but that's the way it is). Boys tend toward sports, wild-life, and violent subject matter. This tends to be the case until about the seventh and eighth grades when there is a more structured art program and the choices by sex seem to begin merging somewhat. At the high school level, there seems to be (now at least, moreso than in the past) a tendency for girls to be attracted to traditionally male subject matter, though where animals are concerned, they prefer pets to wildlife, and female athletes to male. Boys show little interest in traditionally female subject matter.
At the adult level is where I think times have changed the most. Twenty-five years ago female subjects included pets, still-lifes, florals, landscapes, and portraits. Male subject matter then often included wildlife, landscapes, portraits, sports, and large antiques (planes, trains, and automobiles). Today I find few women interested in still lifes or florals at all. They have adopted the male fascination with sports figures, automobiles (though more recent vintages), and quite surprisingly (to me at least) historic, architectural landscapes. There is a much greater desire to mix media today than in the past, and women are more likely to do so than men. Also, Women seem to have fallen in love with abstracts in recent years more than men. Of course these observations are based largely on the work of amateur or semi-pro painters but I think from my past experience, they carry over pretty well into the professional realm too.