Maybe the kid is annoyed with his status as Peanuts Resident Musician. I love his annoyed expression in panel 2, his patronizing "plink plink" in panel 3, and his focused, furious look and how he's thrown in the air with the effort of his playing in panel 4. I think Schroeder's fury here must somewhat mirror the effort Schulz himself put into the strip.
Let's talk a little about how the characters changed over time.
I really can't believe how much the Peanuts guys (in my opinion people who are serious should not use the word "Peanuts gang" to describe them) change in the first few years of the strip. They've already began edging towards their later appearances here. What's odd about them is that the characters move towards becoming less cute and more iconic.
Compare how the Peanuts characters evolve to the evolution of Garfield. Jim Davis, the creator of Garfield, used to be an assistant artist on the bizarre-looking strip Tumbleweeds, and maybe a little of that comes out in the very earliest Garfield strips, which have markedly different-looking characters. Arguably the characters became cuter over time, and that helped the strip to gain traction with readers. But going the other direction, becoming less cute, giving his characters less-over, more circular heads, pushing their stylization beyond the point of maximum attraction and making them still more stylized, that is a strange choice to make.
Right here is, to me, about as cute as Peanuts characters would ever get. The attribute of this style that fixes it in my mind is the expressions on the characters' faces, especially Schroeder here. Eyes wide apart, and with long eyebrows almost mirroring the mouth line. I think this general style continues on later, especially on characters like Lucy, but it's not as balanced, compositionally, as it is in these strips.
Why did Schulz abandon this look? It might have to do with how much time it took to produce. Line thickness, and even the precise thickness of the eyes, is very important to the look.
The more modern versions of the characters tend to have lines that are the same width. The eyes are not just dots but little ovals, and become thick commas when the character is looking around.
And just look at how rounded the character's heads are; there's not a tremor anywhere, it's perfectly smooth every time, the same curve no matter how the characters are facing or posed. That takes skill, and probably at least a little time. (One must wonder how Schulz must have felt about it later in life when hand tremors forced wavering into the perfectly round head of Charlie Brown; even now after Schulz's death, all official depictions of the characters continue to include those tremors.) Also, the characters look very fifties in these strips. If the strip continued on in this style, the characters probably would have turned into something like Calvin and associates.
In terms of the long-term health of the strip it's probably a good thing that it changed. There is still a lingering perception that Peanuts was about cute, and trite "Happiness is a Warm Puppy" sentiment. The move towards less cute, more abstract figures would help the strip as it picked up intellectual depth as the years passed.
