You don't get much more serious than a dead mom, an imprisoned dad, and a homeless kiddo. Even when Sasha gets some time to just relax and chow down on a carrot, we don't get a lot of lightheartedness in the deal:
I take small bites of the carrot to make it last; the carrot is delicious [...] I wonder what it's like for the capitalist countries. I wouldn't be surprised if children there had never even tasted a carrot. (3.2)
So, even though Sasha is enjoying a "treat" (3.1) that is "delicious" (3.2), the author keeps things on the serious level by focusing on the politics of eating. A carrot isn't just a carrot in Soviet Russia. It's a somber reminder of the wonders of Communism. Or something.
With all of the paranoia and arrests going on, there's a danger of things slipping into the absurd, which they never do. Take the moment when Vovka attacks Nina Petrovna. It's so over the top that we might expect to find some humor buried there, but it's all quite deadly serious and gruesome. Nina's "face turns red" and she "makes gurgling noises" (22.12). Yikes.
Despite all of this seriousness, though, Yelchin's tone is also hopeful, thanks to Sasha's irrepressible idealism. Seriously, it's tough to squash this kid's positivity. He's so hopeful, in fact, that he heads straight to prison to visit his dad, even though he has little chance of seeing the guy, and he knows his father handed his mother over to the cops. Is that hopeful or naïve? Maybe it's a little bit of both.