Little Lord Fauntleroy (1980)

Ceddie, Earl of Dorincourt's only grandson and heir, lives in America with his mother. The Earl, getting old, asks them to come to England. Ceddie, now Lord Fauntleroy, is an adorable little fellow. The Earl, who at first was rather distant, becomes more and more fond of him. Then Minna shows up.

She claims she was married to the Earl's eldest son and that her son, being their child, is the Earl's true heir.