The Lost Flower Children
Janet Taylor Lisle1999

Motherless Olivia and her younger sister, Nellie, go to live with elderly Great-Aunt Minty, who knows almost nothing about children, but a lot about the big overgrown garden off her side porch. Then one day a blue teacup turns up in the dirt, and Olivia finds an old story about eight children magically transformed into flowers. Only someone discovering their lost teacups can bring them back. Now the two sisters know what they must do...