Little Lord Fontleroy by Frances Hodgson Burnett

From chapters.ca:
The story concerns an American boy who at an early age finds that he is the sole heir to a British earldom and leaves New York to take up residence in his ancestral castle, where, after some initial resistance, he is joined by his middle-class mother, "Dearest," the widow of the late heir. His grandfather, the Earl of Dorincourt, intends to teach the boy to become an aristocrat, but the boy inadvertently teaches his grandfather compassion and social justice and the artless simplicity and motherly love of Dearest warms his heart.