Elmer’s work suffers from several historical anachronisms. He is old enough that this is just carelessness, not ignorance. In wartime 1940s, household telephones would not have been common. And children did not use backpacks for school until much later; they used satchels or just belt straps.
The main character, Peter, seems incurious; odd for any healthy child.
There are a couple more silly details in the story. After the roundup has started, and the Jewish neighbors are gathered at the Andersen apartment, several spend the day looking out the windows. Uh, NO.
Other than the 11 year old twins carting a wooden boat silently out the door and down a steep ramp in the night... well, the rest of the story is fine.
Except why didn’t they send the pigeon back right away with news they reached Sweden?

I must take back the criticism of the phones. See:
https://www.history.com/news/wwii-danish-jews-survival-holocaust

In 1943, when the Germans plan to send all Danish Jews to prison camps, Peter and Elise, eleven-year-old twins, face danger trying to help their Jewish friend Henrik escape to Sweden.