The Secret Garden by Frances Hodgson Burnett

Mary Lennox is a spoiled and bad-tempered 10-year-old girl who is orphaned when her parents die of cholera. She is sent to live with her uncle, Archibald Craven, at his Yorkshire estate, Misselthwaite Manor. Mary also meets Colin Craven, her sickly cousin, and Dickon, a boy who helps Mary and Colin in the garden