In the examples provided, as well as every other strip that I can find, there's
nothing inconsistent with a young child who has a very vivid imagination.
My personal opinion is that when Hobbes does many of the things that Calvin credits
(or blames) him for, Calvin is actually pretending that he's Hobbes.
The issue in the comic strips is that, when we see the "living" Hobbes, we're
either seeing the other reality that nobody else sees (in which case Hobbes
really is alive and is Calvin's active co-conspirator), or we're inside Calvin's imagination,
in which case, how much of the rest of the scene is also his imagination? Note that the fact that it's his imagination doesn't make it any less real, but it's a subjective reality, rather than an objective one.
We don't know that Calvin didn't have the opportunity to take the cookies; he
could just be very very sneaky.
I've watched children, while pretending, do things equally as elaborate to set
up a simple scenario and then maintain that their imaginary friend helped them,
they "flew up here," or given any of a dozen other explanations. Again - if
Hobbes is real, then he helped. If Hobbes is pretend, then what else is
pretend in the scene?
Simple deflection. Who's to say that Calvin didn't sneak in for a piece of
cake earlier and then pretend (even to himself) that Hobbes did it.
Somebody else commented that this could just be an elaborate explanation for
a wipeout. Makes sense.
Rationalization for falling down, or what he imagined (wished?) would happen