My DD hasn't ever had it cross her mind that girls might not be good at science/math, because it seems patently ridiculous given her understanding of the world. Her mom is a scientist, as are several of our female family friends.
I have not yet talked to my 9yo son about whether there are sex differences in math and science ability, but scores for the contests such as the AMC 8 or for talent search participants taking the SAT are broken down by sex, and he has noticed that there is a difference. Like father, like son

.
And see, my pragmatic child (also like her same-gender parent, I suppose) looks at that same data set and recognizes that there is an
opportunity there... you know, since relative gender disparities from any set of causes means that those who do shine are facing less competition for opportunities. They are that much more desirable for not having a lot of peers following in those same footsteps.
She doesn't see "you can't because you're a girl," she sees "isn't it cool that you'll be that more more desirable by virtue of relative scarcity?"