During the first day of the comic, it's not unreasonable for the kids mostly to be in survival mode (and thus, putting grief to the side), and after they get time for realizing their parents are dead, we don't actually see them until a year later, which is enough time for them to have potentially come to terms with things.


As for the destruction of their universe... Actually wrapping your head around what that means, and feeling it, not just understanding it, is in and of itself a challenge. Frankly, I'm not sure how a human would react to that.