George Washington had no biological children but was a father figure to many.
It's never been definitively established why the Washingtons couldn't conceive—theories range from George's early bouts with smallpox or tuberculosis to Martha's case of the measles. But when Washington married Martha Custis, a wealthy young widow, he became the legal guardian of her two younger children: four-year-old John Parke Custis (known as Jacky) and two-year-old Martha Parke Custis (known as Patsy). He was extremely fond of them, and was bereft when 17-year-old Patsy died of an epileptic seizure.
As a father figure, he was especially fond of dispensing advice via letters—on everything from education to romance. He nagged his stepson to have more discipline with his studies, and warned his granddaughter against marrying for the wrong reasons: "Love is a mighty pretty thing; but like all other delicious things, it is cloying," he wrote, adding that it is "too dainty a food to live upon alone."