Among the Betrayed -- Margaret Peterson Haddix

So.  Back when I wrote up the second book in the Shadow Children series, I complained that while I didn't give a rat's ass about the main character, I continued reading the books because I was hooked on the story. 

The third book is about a completely different kid--a girl, Nina.  Nina was one of the two children arrested at the end of the Among the Impostors.

She finds herself in a cell with three other children.  Privately, she's told that if she doesn't get them to admit to being thirds (the Population Police only allow families to have two children), she'll be tortured and killed.  When she has the chance to escape, she has to make the choice to run alone or take the three others with her.

I totally get why kids end up addicted to this series.  They really aren't super fantastic or anything, but they move fast and have lots of action, along with a mystery/conspiracy element and they (for the most part) pit kids against grown-ups, which is always enjoyable.