What is the ‘childish longing’ that the poet refers to? Why is it ‘vain’?

Explanation

The poet thinks that the persons who are running the roadside stand, suffer from a childish longing. They are always expecting customers and waiting for their prospective customers. They keep their windows open to attract them. They become sad when no one turns up. They are always waiting to hear the squeal of brakes, the sound of a stopping car. But all their efforts go in vain.