I saw this recommended as a wonderful read on a blog a while back and put it on my wishlist. It was a decent story, but to me it wasn't great--it's probably one of those books that makes a big impact if you are 10 or 11 and read it at just the right time.
Waif is a girl on her own. She can do small tricks, which she has to hide so as not to be taken for a witch, and she is directionless until she gives shelter to a woman who has the knowledge of herbs. This woman teaches Waif her knowledge, and before long Waif has a real name and a real job in the castle. That comes with a lot of peril, though, and soon she has to flee, taking her young charge with her.
It's all set in the medieval era, though one where magic and changelings are real. One great element in this book is that the setting is quite realistic--Waif is essentially homeless, and she is cold and hungry and dirty most of the time, and so are lots of other people. It all feels right, which I really appreciated; it's not prettified.
A pretty good story which is worth reading or giving to your kid if it happens to be at your library.