This is such a fantastic piece of fiction. I feel like Maggie and Kelly have already expressed everything that I wanted to say, and much more eloquently than I ever could. Both Severus and Peter are absolutely spot-on: you took the boys we have glimpsed in canon and extrapolated them into such real, painfully adolescent characters. Snape gets paired with the three other Marauders fairly often, but hardly ever with Peter, which is odd, because they are very much complementary characters, even physically: dark, angular, gawky Severus and short, round, fair Peter. Severus, who displays his raw anger in the open, just like he displays his fascination with the Dark Arts, and Peter, who chooses carefully which aspects of himself, of his powers, he displays in front of whom. He shows Severus how skilful he is in many respects, and yet he has no problem concealing his Animagus skills from him. At no point do you imply that Peter struggles under the weight of the secret, even when he and Severus get closer, which strikes me as very true. He wouldn't.
The leitmotif of nothingness is very clever and it is beautifully executed. These boys truly inhabit the world of magic, and the theme of Transfiguration spells is skilfully interwoven into the story, driving character development and plot alike.
I like the cast of supporting characters, as well. Sirius in particular stands out for me, the repressed violence that threatens to burst out at any moment is palpable. And the atmosphere is beautifully crafted, as well: the sweltering summer heat, the adult-free world in which your teenage protagonists live, all this is very reminiscent of classic children's adventure stories. Albeit darker.