Since the requested summary prevents you from giving away the ending, the summary is more of a teaser summary that attempts to get your reader interested in reading the book for himself or herself.
Some things that your summary should include are setting and characters. Let your readers know that The Boy in the Striped Pajamas begins in Germany during the early 1940s. Hopefully your reader is aware that setting places the book during World War 2, but you are certainly welcome to specifically mention that conflict as well.
Next, include information about the book's main character, Bruno. Tell your readers that the book begins with Bruno and his family moving out of Berlin into a much smaller home that is next to some kind of facility surrounded by high fencing and guards. Let your readers know that Bruno's new home is next to Auschwitz, and inform your readers that Bruno is incredibly unhappy there. He has no friends there until he meets Shmuel.
Tell your readers that much of the remaining story is about how Bruno and Shmuel become closer as friends. They have moments where the trust is betrayed, but all in all, the friendship is good for both boys.
Next, leave your readers with a cliffhanger that tells them about Bruno sneaking under the fence to help Shmuel look for his father. This is where your summary should end and say something about the experience being an incredibly dangerous one.