Why is the boy being punished in "The Lumber Room"?

The young boy, Nicholas,
is being punished because he refused to eat his milk and bread in the morning because, as he
said, there was a frog in it. Many adults assured him that this was not possible and that there
was no way a frog could be inside his bowl, but he continued to insist upon it. In truth, there
actually was a frog in his bowl of milk and bread because he, himself, had
taken it from the garden and put it there.

This incident proves to the boy
that adults can be "profoundly in error," even when they have expressed absolute
certainty on a subject. One of these adults, the woman who calls herself his aunt though she is
not really a relative of his, actually devised the special trip to the beach at Jagborough so as
to really make Nicholas feel the seriousness of his bad breakfast behaviorbeing barred from
going to the beach is intended to guarantee more favorable behavior from Nicholas in the
future.

He is likewise forbidden to go into the gooseberry garden while his
cousins are gone on their trip. With his "aunt-by-assertion" standing sentry at the
gooseberry garden, Nicholas is free to steal a key to let himself into the lumber room and
explore it at leisure.