1970s book. Boy or girl goes to live with aunt, who is a good witch. The first thing they do is have tea; pastries grow from a plant in the center of the table. Kid meets another kid; they work to fight an evil witch, at one point, with rowan berries (“Rowan berries and red thread stop a witch in her speed”). They must go to a magical place, where they see a silver tree, then a golden tree, then a diamond tree. That’s pretty much all I remember. Thanks!