It starts dramatically enough. On their way to a new life in Australia they get shipwrecked somewhere in the East Indies and the crew abandon them. But luckily the ship doesn't sink and they're within swimming distance of a beautiful island which has everything you could want to survive, including running water and a geographically impossible abundance of food and livestock. The ship conveniently remains intact long enough to get all its goodies onshore (more livestock, tools, chest of gold, you name it). They build a fabulous treehouse, including a front door and a staircase up through the trunk. They use rubber trees to make shoes and waterproof coats. There are a several light-hearted adventures, hardly a moment of hardship and Sundays are a day of rest. They live there for 13 years. Nice.
But we mustn't be too critical I suppose. Wyss wrote it for his sons as a series of lessons about family values and self-reliance, and it resembles other early children's books of the 19th century. Of course it's got Disney written all over it, and they obliged with a suitably wholesome 1960 film starring John Mills.