Is this book appropriate for 4th graders?

My 9 year old son was recently assigned the book "Among the Hidden" by
Margaret Peterson Haddix. It is set in the future when population
control is a big enough problem that families are only allowed to have
2 children. Third children (like the main character) must be hidden
away to protect them. There are a lot of parallels in this story to
China, for example, in a protest 40 children are shot and killed (like
tiananmen square). It is a small part of the story, but it packed a
big punch when he read it.

My son was very upset after reading the book. I think the subject may
have been a bit too heavy for him. He is just a very sensitive kid. I
wish the teacher had warned us what was coming in the story so we
could have prepared him for it.

What are your feelings on assigning books with heavy subjects like
this to 9 year olds? I'm sure some kids handle it better than
others. Is it too much to ask to have the teacher forwarn us about
upsetting themes?

Thanks for your input.

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