Robin by Frances Hodgson Burnett

Robin sat at the desk in her private room and looked at a key she held in her hand. She had just come upon it among some papers. She had put it into a narrow lacquered box when she arranged her belongings, after she left the house in which her mother continued to live. It was the key which gave entrance to the Gardens. Each householder possessed one. She alone knew why she rather timidly asked her mother's permission to keep this one.

"One of the first things I seem to remember is watching the gardeners planting flowers," Robin had said. "They had rows of tiny pots with geraniums and lobelia in them. I have been happy there. I should like to be able to go in sometimes and sit under the trees. If you do not mind -- "