This is a rather sweet story about a mouse who loves a princess. Though written for the younger set (at 6, I'd say my kid was just the right age), there are some rather brutal moments: Tails are chopped off, parents die, children are beaten so badly that they go partially deaf, someone dies of fear... The book doesn't really hold back, and I can see some parents (and perhaps more sensitive children) being put off by it. 
 As it is, the central message of the book is one of empathy. Having most of the villains be hurting, and even the "good" characters doing bad things because of their grief, generated a number of good teachable moments about that empathy theme.