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

Hint: The lives of impoverished, disadvantaged individuals are the subject of A Roadside Stand synopsis. Furthermore, the poet compares the hardscrabble lifestyles of rural people with the callous lives of city inhabitants.

Complete answer:
City inhabitants are unconcerned about the poor living conditions of those who work at roadside stands. City inhabitants are unconcerned about the difficulties these roadside vendors face in selling their wares. These poor folks have nothing to do but wait for passing automobiles to come to a halt and buy their wares. If a vehicle comes by, it is either to ask for directions or to complain about something. The poet empathises with and feels compassion for these underprivileged folks. This affection is obvious in the sad depiction of the wayside sheds.

The poet alludes to individuals who work at a roadside kiosk as having a "childish yearning." They naively hoped that automobiles would come to a halt in front of their stand. However, their yearning is in vain because they receive very few visitors. People don't even bother to inquire about costs. The automobiles that come to a halt at the stand do so to turn around, ask for directions, or request items such as gas, which the stand obviously does not sell. The poet recognises that the self-centered city dwellers are unlikely to take notice of the impoverished people's predicament, let alone think about it. Thus, the 'childish' longing that the poet refers to, is the longing of these rural people that one day at least someone of all the thousand selfish people who pass in their luxurious cars, will halt even if it is only to inquire about a farmer's miserable plight.

Note: Robert Frost's poem 'A Roadside Stand' deals with the life of impoverished disadvantaged people in villages with a penetrating clarity while also portraying his innermost sympathies and sentiments of compassion.