The poem catches a single moment: a man hears from his wife that she thinks she is pregnant. Before that news arrives, the opening lines sound like a worry about scarcity. There is "no other money," "no other time," and nothing to fall back on. The phrase "all alone" is a little ambiguous, but it seems to describe how a person feels when responsibility is about to land on him.
The speaker's reaction isn't spoken. He doesn't say anything to his wife. Instead he flings off his hat and sits with the wind, as if he needs to be outdoors, bareheaded, and still for a moment. That gesture could mean shock, relief, panic, or a kind of wild openness, and the poem doesn't choose. The title, "Grownups," hints at what's happening: he is realizing he's about to be one in a new way.
The mood is quiet, tense, and a bit exhilarated, with fear and wonder sitting side by side.