Voyage of the Unicorn (2001)

Mourning the loss of her mother, nine-year-old Cassie Aisling daydreams of the magical, mythical world her mother left unfinished. Then, one day, a troll appears. Then another. And another. Before they know it, the Aislings find themselves in the vortex between worlds and transported to a ship called the Unicorn. Here, the Aislings discover that they’ve been chosen to find the magical dragon.