You will need:
Soda water contains a large amount of dissolved carbon dioxide. Pouring the soda water into the container helped to release this gas in the form of bubbles. The bubbles are less dense that soda water, so they rise to the surface.
The bubbles connect themselves to the sides of the sultanas, providing a buoyant force to help to raise the sultanas to the surface. On reaching the surface the bubbles pop, thereby reducing the buoyant force on the sultanas and they fall back down again.
Marine archaeologists fill bags with gas so they can raise shipwrecks and sunken treasure!
Similar experiment: Make a simple lava lamp
Have us visit your school! The working with water school workshop is designed for younger students to get their head around why things float and think.