How is the theme of perseverance depicted within Chapter 9 when the Small Foxes make Mr. Fox proud?

The theme of perseverance emerges within Chapter 9 when Mr. Fox comes up with an idea to save the family, but he declares that he does not think it will work since everyone in the family is too tired and hungry to continue the digging that his plan will require. Much to Mr. Fox's delight, the Small Foxes excitedly exclaim that they would be happy to help, demonstrating their tenacity and determination to survive.