Well, to be fair, this is an old story, it was told over 100 years ago! (1800's!) when ideas of immunity were new to the general public. There is actually a very interesting story behind the original RADIO broadcast of this story that you should look into. Now as for the daughter, she was actually a great little actress, and I think having her freaking out and screaming would actually be quite accurate for a little kid in such a scenario! They are so vulnerable and useless, all they can do is scream, run, and hide! And she did all three! The son is angry and rebellious, totally natural. He wants to fight because his options are either be at the will of his estranged father, or defend his country, which is a much more noble cause. As for the birds, it's not that they could do something weapons couldn't, it's that they indicated that the impenetrable shields were down, when everyone assumed they remained, which informed the military that their primitive weapons would work against the aliens now. For me, the most improbable element of the story is that the aliens weren't aware of germ theory, but again, since the story is so old, at the time of it's writing, most people weren't aware of germ theory! It's the same issue with the movie Independence Day. Back then, the idea of "computer viruses" was new, so the audience was learning about this just as the aliens were. At the time the idea made total sense, it's as if the aliens and the humans at that time are in the exact same paradox, even though the aliens have superior technology. In both stories, it's as of the aliens are providing an alternate perspective on a perceived disadvantage facing humans. It's as if to shift our frame of reference so that we see a disadvantage as an advantage.