The Secret Garden by Frances Hodgson Burnett

Mary Lennox is a sour-faced 10-year-old girl, who is born in India to selfish wealthy British parents who had not wanted her and were too wrapped up in their own lives. She is later sent to Yorkshire, England to live with her uncle, Archibald Craven at his home called Misselthwaite Manor. While exploring the gardens, Mary comes across a badger hole and finds a key belonging to the untended garden. She chances to ask Martha for garden tools, which Martha has delivered by Dickon, her twelve-year-old brother. Mary and Dickon take a liking to each other, as Dickon has a soft way with animals and a good nature. Eager to absorb his gardening knowledge, Mary lets him into the secret of the garden, which he agrees to keep.