Throughout "The Selfish Giant," the tone remains—for the most part—matter of fact, as the speaker recounts events as straightforwardly as possible. This is in line with the story's genre, as fairytale narrators tend to be relatively removed third-person speakers.
After the Giant forces the children out of his garden and puts up the notice board saying that "TRESPASSERS WILL BE PROSECUTED," the speaker tells the reader, "He was a very selfish Giant." This is one of the only moments in which the speaker seems to have been impacted by the characters and events of the story to the point of expressing some kind of personal opinion about what is happening—that is, the speaker interjects in this moment with a disapproving, somewhat moralizing tone. As the story goes on, though, Wilde's speaker strictly narrates the trajectory of the story's events instead of providing commentary on them.
Although the story includes a number of dark and touching moments, the speaker's tone does not seem to change with these plot developments or shifts in mood. Whether the Giant is cruel or kind, whether the garden is in deep winter or full summer, the speaker narrates with a neutral tone.