There isn’t too much to be concerned about here from a morality standpoint (man, we love saying that). The primary consideration revolves around the natural intensity that often comes from historical fiction. Below is the information to consider for your young reader.
The biggest consideration point revolves around the setting. It never gets too intense, but characters are in the middle of an invasion by an evil empire and the story heads into the conflict and escape. We read a few lines like the following:
When Max shares that he knows his parents, who support an underground effort against Hitler, being caught would “certainly mean execution”.
To help us understand about Hitler’s cruelty we are told “Men, women and children murdered for any reason, no reason at all”.
“Max knew at that moment he was going to die”
“Wouldn’t hesitate to murder the both of them right there in their doorway”
“I watched them beat him” (boy speaking of his dad)
“It was the boom of guns. Big guns. And bombs. Men killing one another and being killed.”
“Bombing is called a “chorus of death and destruction”.
A soldier is shot and there is a reference to his blood. (we will mention this is an exception as, knowing his audience, even the enemies are typically shown to be ok)
Here are the other things we noted to share:
Several reference to luck (“if luck's on our side”). The worst of these is probably “He’d have punched his ticket to the great beyond if not for a little luck”.
A place is called a “godforsaken beach”
An evil general is rumored to have a cain that was “carved by the devil himself”
“For all Max knew, they might be heading towards an underworld city of the dead.
A character wonders “which would be worse, drowning or being buried alive?”
A character remarks “I don’t see any reason he needs to know about any of this” in reference to concealing something from her grandfather. Similarly a girl is asked if she is supposed to be in her father’s study, to which she replies “Not exactly. But what he doesn’t know won’t hurt him”.
Sadly a girl refers to a good idea as “truly diabolical”.
In a line that is supposed to be innocent curiosity a young character asks, “What do you think people taste like?’