Symbolism, Imagery, Allegory
Dream a little dream…of the sun? Hm. That doesn't seem quite right in a book about snow, but you can bet Keats had a reason for including it.
Dreams are often symbolic, and Peter's dream is no exception. While dreams—as anyone who's had one knows—can sometimes come from out of left field, Peter's makes perfect sense. After a wonderful day of adventures in the snow, Peter suffers a disappointment just before heading to bed. The snowball that he had tucked away for tomorrow has melted, making him extremely sad (33).
It only makes sense, then, that as he drifts off to sleep, Peter could be concerned about losing the rest of the snow as well. And when he dreams, that exactly what he dreams about: the sun coming out and melting all the snow away (34).
Peter's dream can be seen as a symbol of Peter's fear about losing his beloved snow, but it can also be seen as symbolic of Peter's—or the reader's—fear of losing youth. Snow is beautiful to people of all ages, but it's particularly magical for children. So a world in which the magic of a snowy day is erased overnight by a hot sun is a world which has lost a bit of its magic. And a world in which Peter, who's already had to grow up a bit due to his melted snowball, would have a bit more growing up to do in dealing with this, much larger, disappointment.