Why were each of the characters silent during evening tea?
At the beginning of the story, we are told that the children will have a day at Jagborough Beach. We can probably imagine any child's excitement at the prospect of such an outing. However, the author foreshadows the somber mood of the evening by stating that the party departs 'without any of the elation of high spirits that should have characterized' such an expedition.
Immediately after the children leave, Nicholas asserts that Bobby probably won't enjoy himself because of his ill-fitting boots. His aunt is incredulous and demands to know why Bobby didn't see fit to acquaint her with this information before he left. Nicholas answers that Bobby did mention it twice to her, only to be ignored each time.
At evening tea, true to Nicholas' prediction, Bobby is sulky. His ill-fitting boots definitely ruined his temper during the afternoon; at this point, we can infer that he probably wasn't a very engaging playmate at the beach. The other children are equally unhappy because they never obtained the opportunity to run on the beach due to the high tide. Nicholas' aunt is shell-shocked and noticeably chastened by her unfortunate experience in the water tank. Meanwhile, Nicholas is enjoying the private reveries of one who has enjoyed an afternoon of stolen delights and who has triumphed over an enemy.