Exploitative child labour is a dark theme for a middle grade audience, but I believe it's worthwhile for younger readers to explore grim settings. Perhaps the intention is to evoke compassion in the reader, which is definitely one of the book's key themes. 

My only criticism is that the story covers its topics superficially. I wish that it had explored the grief, defence mechanisms, and empathy a bit more.

However, I really appreciate that Bess and Dot have a [somewhat] happy ending! I was so stressed because I thought Dot would be killed off.