An "incident" is something that has happened in the book -- one of the events of the book. Take some time to look at the incidents that affected Bruno.
1. Bruno was seeking a tyre to make a swing. Lt. Kotler found Pavel, an old man dressing in the striped pajamas. The lieutenent talked to Pavel in such a way that Bruno was ashamed. Then,later, when Bruno fell fom his tyre swing,Pavel came to his aid and that fact had to be hidden by his mother. (Chapter 7)
2. Bruno first meets Shmuel. (chapter 10)
3. The night the Fuhrer came for dinner. (Chapter 11)
4. The Christmas that his grandmother walked out of the house after an argument with his father and never spoke with his father again. (Chapter 8)
5. Pavel being beatenby Lt. Kotler at dinner. (Chapter 13)
6. Bruno denies that he knows Shmuel and suffers extreme embarrassment. (Chapter 15)
7. The final adventure that Bruno and Shmuel plan and the outcome of that adventure. (Chapter 18-19)
Each of these is a major incident leading to the final days of Bruno. There a many minor incidents also. I hope this helps.
Thank you very much, this has helped immensely(: