Children are dumb to say how hot the day is, How hot the scent of the summer rose, How dreadful the black wastes of evening sky, How dreadful the tall soldiers drumming by.
At first, it's gonna look like Robert Graves's speaker is being mean and picking on children for being "dumb." But what he actually means here is that children are silent or unable to speak. More specifically, he's saying that children aren't able to say "how hot the day is" or "How hot the scent of the summer rose" is.
It's weird to think of a smell as being "hot," but Graves is messing with our idea of how our five senses work together. It's like when you say that someone's colorful shirt is a "loud" shirt. You're actually substituting one sense (hearing) for another (the sight of the shirt). This is a little trick called "synesthesia."
In any case, we should all be wondering by now why children aren't able to talk about how the day is. But this is all child's play at this point (pardon the pun). In line 3, Graves gets a little darker and talks about how children are also unable to talk about how "dreadful" the black, evening sky can be. As far as we know, there's something about children's minds that doesn't allow them to talk about the things they see.
Finally, Graves takes all of his nature images (the hot air, the roses, and the dark sky) and introduces something more political and historical in the image of the "dreadful […] tall soldiers drumming by." After mentioning that kids aren't able to put their experiences into words, Graves also says that these children aren't really able to describe or understand the meaning behind a bunch of marching soldiers.
But do you know who can understand the meaning of these soldiers? Robert Graves, a man who was severely injured and traumatized by his role as a soldier in World War I, which would have ended less than ten years before Graves wrote this poem. Of course, it's never a good idea to get the speaker of a poem twisted with the biography of the poet him or herself. In this case, though, in mentioning the soldiers Graves is no doubt referring to his own struggles to put his own experiences into words after the war had ended. So, by the end of stanza 1, we can tell that Graves is making a point about how children don't put their experiences into words, and how they don't understand the meanings behind certain things in the adult world (like soldiers marching) in the same way adults do.
One final thing before we skip off to the next stanza: notice any rhyme going on here? Hey, what a coincidence—us, too. Lines 3 and 4 have a matching end rhyme with "sky" and "by." For more on what that might mean, click on over to "Form and Meter," but be sure to come on back.