While I can't complete this assignment for you, I will point you towards two humorous anecdotes in this great book that you could refer to when writing your letter.
For the most part, this book is incredibly somber. It is the story of the Balicki family, the patriarch of which was arrested for daring to turn a poster of Adolf Hitler so that the dictator's face faced the wall. His wife was later taken to the labor camps, but the children managed to escape capture.
The first incident that could be written about with humor is Joseph's escape from Zakyna. Having managed to knock out a guard, Joseph manages to steal his uniform and put it on. His method of escape was as undramatic as could ever be imagined. He simply walks out with the guards, pretending he is one of them. The sheer bravery of this move makes it humorous, and in writing a letter about this, you could imagine how Joseph might have laughed from sheer nervous tension after his escape.
The other funny incident—and this is the one that I would choose to write a letter about—is when the Balicki children have been reunited and arrive in Berlin. Jan, who clearly has a way with animals, tracks down a chimpanzee who has escaped from Berlin Zoo. The chimp, who had bitten a policeman by this stage and was widely feared, shows an immediate liking and respect for Jan. This is great material for a humorous letter.