Saw this probably around 1967-1969. Children discover a cave but to explore they have to build a raft/boat. They get help from local adults who help them, but can’t understand why they want to build a raft when the nearest body of water is hundreds of miles away. Later, of course, they have to hide in the cave. I think of it as Australian, but it could easily have been set in America. I’ve confirmed it’s not “They Found a Cave” (1962).