The quack chicks are two tiny cheeping yellow bundles of feathers who are discovered by Will Venables in a pocket. They are of a breed of waterfowl native to the world he lives in -- Thule -- and he didn't realize when he left his world that they were there. While the main characters gather in Rupert Venables' hotel room to prepare to perform a major magical working, they scurry around the room pecking at biscuit crumbs that people scatter for them to eat. At several points in the story they precipitate some bit of the plot that the characters themselves didn't foresee happening that way.
And when the working is underway, they surprise everyone by waking up from a nap and dashing out into the working and out of sight, which leads to a very unexpected outcome for all concerned.