I agree that it's ironic to see adult characters being childish and children being worldly adults and it's based on the statements below. In some stories, adults are illustrated in childish character so as to reveal some phenomenon and in the society, such as the idleness of the upper class and man's pride. Archibald and Aurelia in 'The reverent wooing of Archibald' and Johnson from the 'Dream Cargoes' are the examples. On the contrary, the portrayals of children in some stories are worldly adults in order to bring out themes that are war-related, which are also some social phenomenon. Cases to be considered are Trevor in 'The Destructors' and Eric and Bertie in 'Toys of Peace'.
Despite the nurture of Eton and an upper class family, Archibald remains idle and gullible. He is not knowledgeable and behaves as silly and immature as a child. According to the narrator, he is the 'soul of a hen-imitator' and 'the master at imitating a hen laying and egg'.
His childish, ludicrous performance is described in details in the climax of the story, seems to show how 'professional' he is, and yet, the narrator actually shows the masked writer's sarcasm and mockery towards Archibald. His gullibility and stupidity can be seen when he believes Aurelia's Aunt's 'Bacon Theory'. Not just Archibald however, has childish behavior in the story. Aurelia, the goddess of Archibald, who is an upper class lady, is fascinated and amused by his childish and foolish 'hen imitation'.
The writer uses childish Archibald as a type, which represents idle people who belong to the upper class. Through the illustration of Archibald's life, such as hanging around bars, clubs and collecting socks, the writer criticizes the upper class people that remain idle and are not productive or have contribution. It's ironic...