the apple of somebody's eye

the ˌapple of somebody’s ˈeye

a person, usually a child, who somebody loves very much; a favourite child: The second child, John, was the apple of his mother’s eye.In the past, the pupil (= the small round black area at the centre of the eye) was called the ‘apple’ of the eye, the part that had to be protected most carefully.
See also: apple, eye, of