Between the Trees by Ayn O'Reilly Walters
Twelve year old Isabel Pritchard lives with her sixteen year old brother, William, and their mother. Behind Isabel's home is a small park and when Isabel finds herself late home from school one afternoon she decides to take a short cut through the park, knowing that her mother has forbidden her to enter the park. It's a decision that changes her life as she uncovers family secrets that go back five hundred years, all the way to the birth of Queen Elizabeth the First.