By their very nature, any book about the horrors of the Holocaust and World War II are not humorous. However, there are various anecdotes in this great story that can be argued to be humorous.
The first of these, in my opinion, takes place when Joseph manages to escape from Zakyna, a prison camp, by successfully stealing a guard's uniform, putting it on, and sauntering out without a care in the world. He informs the boss of the outfit's true owner that he has nothing to report.
Later on in his mission to escape, Joseph is captured by a Polish man who later gives him refuge. The first moments of their encounter are humorous. Joseph "threatens" him by pretending that a chocolate bar is a gun.
Much later in the story, Ruth, Bronia, and Jan leave Warsaw and head to Posen to find Ruth and Bronia's brother, Edek. They also want to find the transit camp where they heard he was being held. The travelers would have made a comical picture, as they were accompanied by Jan's chicken, Jimpy.
When the siblings are reunited with their brother Edek, he tells the humorous but improbable story of his escape from prison camp. He escaped by clinging to the bottom of a train and getting frozen to it after the train went through a big puddle.
Another humorous moment comes while the children are staying in a transit camp. Hilarity ensures after a chimpanzee escapes from a nearby zoo and decides to pay the camp a visit.