This reads like a sequel, with the consequent pitfall that the reader is expected to be familiar with the personalities of the main protagonists, who are therefore given little characterisation.
That aside, all of the characters are well written, with some depth given to even the most apparently stereotypical.
I approached this book with some trepidation, as books about slavery for children tend to either a) trivialise the horror for sensationalist purposes or b) fall into stereotypical generalisations. This falls into neither category. Mr. Nicholson has invented a distinctive, individual, and plausible society, with full attention paid to considering how it arose and how it functions.
As a result, he delivers, within the context of an exciting adventure, a thought-provoking study in the problems of government, and the morality of resistance. I picked this book up to assess its suitability as a gift, and was interested enough to read to the end! Expect not only for your young reader to be enthralled by the narrative - you should be prepared for some interesting questions/conversations afterwards.
One warning - take a look at the cages in the opening chapters. The horror level might be too strong for some younger readers.