I thought the 1974 film of Swallows and Amazons was much better.

On holiday with their mother in the Lake District in 1929 four children are allowed to sail over to the nearby island in their boat Swallow and set up camp for a few days. They soon realise this has been the territory of two other girls who sail the Amazon, and the scene is set for serious rivalry.

http://www.imdb.com/title/tt0072233/plotsummary