Assuming that all of them can sail the boat. For them to sail to the island.
First: The two children (Nancy and Peggy) should sail to the island. Next: One of the children (say Nancy) should bring back the boat. Next: One of the adults (say Uncle Jim) should sail to the island. Next: The other child (Peggy) should bring back the boat. Next: The two children should sail to the island. Next: One of the children should bring back the boat. Next: The remaining adult (the mother) should sail to the island. Next: The other child should bring back the boat. Finally: The two children should sail to the island.