Most likely, the main character is the kid. And of course, the Sandman would be another important character. There might also be the kid's family members or friends who play a role in the story, perhaps by being part of the kid's real - life experiences that are then influenced by the Sandman in the dream world.
The central characters would be the kid and the Sandman. The kid is the one through whose perspective we experience the story. The Sandman, on the other hand, is the catalyst for the events in the story. Additionally, there could be a wise old character in the kid's neighborhood who has some knowledge about the Sandman and gives the kid some advice about dealing with this magical being.
The main characters are probably the kid and the Sandman. The kid could be an ordinary child with an extraordinary adventure waiting for him. The Sandman is the mysterious figure that drives the plot. There may be other minor characters like the kid's pet, which could be a source of comfort for the kid in both the real world and the dream world created by the Sandman.