I have been searching for this book for years.  It captured my imagination during childhood, and I must have read it at some point in the 1980’s – may well have been published well before that.  I am sure I have some details wrong, but the plot involves a child coming to visit a large home near a lake for the summer, with an older woman as the guardian for the summer.  This woman had years ago lost her own child, her son, who I believe drowned in the lake.  The details I remember more clearly involve a special lake island playhouse that she built especially for her son – the playhouse had its own furniture, dishes, silverware, etc.  This special playhouse and island was off limits now that the son was dead, and the woman did not want to talk about him.  I believe the dead son’s name could have been Trelawney, or something like that.  The child now visiting for the summer sneaks to explore the world of the island and the playhouse, and falls into some sort of danger.  The ghost boy (? Trelawney) helps save the day.  For some reason I am thinking the word “green” may be in the title, but that could be wrong.  Please help!!  Thank you!