Babies and bombers just don't mix well. You can't get more innocent than a newborn baby. They are vulnerable and oblivious to the evils of the world. The birth/womb imagery that Jarrell uses puts this notion of innocence in the reader's mind. In "The Death of the Ball Turret Gunner," these ideas of innocence are juxtaposed, and then combined with, images of violence, heightening the reader's sense of war's horror.
Despite what the poem seems to lament, innocence is meant to be a temporary state and the sooner our eyes are opened to the harsh realities of the world, the better. So toughen up, kids!
As the poem shows, even after losing an innocent perspective of the world, we long for the comfort an innocent perspective allows and we try to recapture it. (Come on now, we can't be the only ones with a Star Wars figure collection.)