The Castle in the Mist by Amy Ephron

Tess and Max are sent to live with a maiden aunt in England while their father works as a war correspondent in Afghanistan, and their mother recovers from an illness.  Tess finds a old key near the ruins of a castle near their aunt's country home.  The key is magic, and when Tess uses it to open a rusty gate, it leads her into a world when the castle was the home of young Lord William, his nurse, Marie, and a number of house servants.  Tess and Max visit William a number of times and have sometimes enchanting, sometimes frightening adventures.  Both William and Tess have the same wish, that they can be reunited with their parents, and it is up to Tess to make the wish come true.

I really liked this one.  It has an old fashion feel, like the Edith.Nesbit novels of early 1900's.  The children are kind and gracious to each other, and their adventures are fanciful and imaginative.  I was worried that it would have a really sad ending, but Ephron did a good job of making the story feel like it has a satisfying and positive resolution.  (167 p.)