Apple of the eye

a language becomes rich and richer by being used by great bards like Shakespeare. He used this phrase "Apple of the eye" in his great play, "a midsummer night's dream"
Figuratively speaking the child is the apple. Examples may be multiplied some of them are
1. "An apple a day keeps the doctor away"
2. "Child is the father of the man"
3. "Let nature be your teacher"
4. "Doctor's dilemma"
These are all poetic expressions.