In Number the Stars, why does Uncle Henrik say it is easier to be brave if you don’t know everything in Chapter 9?

In Number the Stars, Annamarie is told that they are needing to prepare for her Great Aunt Bertie's funeral. Annamarie confronts Uncle Henrik, stating she knows there is no Great Aunt Bertie and she wants to know why she is being lied to. Uncle Henrik tells Annamarie that it is easier to be brave when you don't know everything. If she were to know everything, it could put her in danger. This becomes true in many situations throughout the remainder of the book and Annamarie begins to understand it more.
The first example is when Peter gives Mr. Rosen the packet with the handkerchief. While Peter knows what is in the packet, he does not tell Mr. Rosen. Annamarie realizes that it is better than Mr. Rosen does not know what is in the packet because it could put Mr. Rosen in danger. We later find out the handkerchief was coated in something to protect the Jews on Uncle Henrik's boat from being sniffed out by the soldier's dogs. Had Mr. Rosen know what was in the packet, he may have been too afraid to take it. It could have also been dangerous for him to know if he had been confronted by someone (like a soldier) later on.
Going back to Great Aunt Bertie's funeral, it helped Annamarie (and Ellen) be brave to not know the truth. Ellen did not know that Annamarie didn't have a Great Aunt Bertie. When Nazi soldiers arrived at Uncle Henrik's house, Annamarie was more easily able to answer the soldiers' questions. She was in a sense, more brave and confident in her answers because she did not know the truth. While she knew there was no Great Aunt Bertie, she did not know that there was not a dead person in the casket.
Another example of Annamarie's braveness is when she needs to deliver the packet to Uncle Henrik. She tried getting her mother to tell her what was in the packet, but Annamarie does not find out before she leaves for Uncle Henrik's boat. When stopped by soldiers, Annamarie realized how important it was for her to be unaware of what was in the packet. She points out she is not lying to the soldiers when she tells them she does not know what is in the packet.
These are three main examples showing times when someone was bravest when they did not know all of the answers.

Source: Lois Lowry. Number the Stars. Yearling Newbery, 1989.

In Chapter 9, Uncle Henrik tells Annemarie that it is far easier to be brave when one does not know everything. What he means is that a lack of knowledge about dangerous secrets or impending threats can help someone keep his/her composure during stressful situations. 
In the chapter, Annemarie is told that a funeral will be held for Great-aunt Birte. However, Annemarie knows that there is no such relative in her family. She is furious that the adults are lying to her. Her anger abates, however, after Uncle Henrik speaks with her. The funeral is actually a cover for gathering a group of Jewish refugees at Uncle Henrik's home. The refugees are to be smuggled out of Denmark in Uncle Henrik's boat in the dark of night.
Uncle Henrik tells Annemarie that he can only divulge certain information to her. In other words, she will be told only what is necessary for her survival. Details that are left out are for her protection: if Annemarie doesn't know the full truth, she will not stumble or seem nervous when questioned by Nazi soldiers. This is why Uncle Henrik tells Annemarie that it is far easier to be brave when one doesn't know everything.