In the book Number the Stars, what was the code used to help Jewish families escape Denmark in 1943?

I believe that you are asking about using code language during a hypothetical situation similar to that of Ellen's and Annemarie's in Number the Stars. 
Let us first add some background as to why code was needed, and how it was used in the novel.
Number The Stars, by Louis Lowry, talks about World War II and how those victimized in society still managed to help one another, particularly in the case of Jewish people escaping from Denmark.
Code language was one of the many ways people used to communicate, especially when the Danish people who resisted the German occupation of Denmark attempted to deliver messages or carry out plans of escape to help Jewish people escape to Sweden. 
Since the way to help Jews escape was by hiding them in boats, the only way to safely help would have been by having a fisherman, such as Uncle Henrik from the Johansen family, prepare the way ahead and convey the message that "the coast is clear," meaning it was safe to bring people for transportation. For this, they used the following code: 

Tomorrow will be a great day for fishing.

Once Annemarie Johansen heard this, she would know immediately that it was a safe time to smuggle the Rosens, or any other family that needed to escape, out of Denmark. 
When it was time to bring Ellen out, Mr. Johansen said that he would send his wife for "a visit" along with "a carton of cigarettes." Annemarie again noticed the weirdness of it, as cigarettes had been out of sight for a while. This is when she realizes it was time to deliver Ellen and help her escape. Ellen was "the carton of cigarettes."
Now, in a hypothetical situation, code would be equally necessary. 
The reality is that one would have to use code for everything during the occupation. All of your actions are monitored, and the Germans completely disregarded the basic rights of any citizen. If you had a friend who was Jewish and you were part of the resistance, you would have needed to change your entire method of communication to divert the attention of the people around you. You would have not wanted any eyes on you; basically you and your friend had to be watching out for anything that was said outside of the immediate circle. You had to learn to trust, and that was surely the hardest part of it all.