While this deals with quite dark subject matter, and is very intense at times, it's also quite an easy read. It doesn't shy away from the reality of the story it's telling but it doesn't linger on the horrors of it, and it's not overly graphic. The juxtaposition of Judith and Moses' POVs, alongside the various adult POVs, work really well to illustrate the attraction of Nathaniel and his Ark to the adults, compared to the instinctual nature of the children, even Moses who was born in the Ark, that things weren't right. 
This is the second of Rebecca Wait's books that I've read and they both dealt with some really dark themes, but in a quite restrained and thoughtful way, allowing the characters to react in natural and believable ways in the face of tragedy and trauma.