Kingdom Hearts: The Novel (2005)

Blurb:
On the Destiny Islands, three children—Sora, Riku, and Kairi—are living out their peaceful, carefree lives while yearning for whatever lies beyond the great ocean. But one night, an unexpected disaster takes place, and the three are torn from each other and their island home. Meanwhile, at Disney Castle, Donald Duck and the other castle residents are in an uproar upon discovering King Mickey has suddenly gone missing. When fate brings them together, Sora, Donald, and Goofy set out on a grand Disney adventure to find their friends!

Source: www.anisearch.com/manga/12397