Hmm. I think there were children who were prisoners. I also have some image of the monsters being pink or purple (but I might have made that up). When I tried to look it up my brain was telling me it was called something like the prisoner of Zenda (which it obviously isn't!), so it might be something similar sounding. Also to complicate matters I think I read it in Swedish so I have no idea if it was translated or by a swedish author.

If someone can actually figure this out I will be amazed!