Answer:
One example of light in the darkness is the friendship we see evolving between the children. Bruno and Shmuel interact with one another despite a barrier. A beacon of hope which makes their time during that period more bearable. It gives us a glimmer of hope as a audience that despite their horrible surroundings they could find an ounce of happiness with their interactions.
Another light in the darkness moment is just how the children are that children. They are in retrospective, innocent of adult thinking. If you think about it, an adult has placed these "people" into these camps but most children are innocent of these types of thinking. They don't see people are indifferent. So, there interactions with one another is one of pure innocence. Unlike their parents they harbor no hate for each other. They simply see one another as two boys across a fence with perhaps separate lives but why? This is the beauty of child innocence.