What reason does Mr. Fox provide to the Small Foxes when they ask why he cannot tell them their destination?

When the Small Foxes inquire of their father the reason why he cannot tell them their destination, Mr. Fox explains his reasons. He says that if he were to tell the Small Foxes about their destination, they would become too excited to keep digging. He also says that if for some reason they could not reach the place he had described, then he is afraid that the Small Foxes would "die of disappointment" (37).