expression
apple of one's eye; dote on someone
Idiomatic expression describing extreme fondness, often for a child or grandchild, implying that even putting them in one's eye wouldn't hurt. Used hyperbolically.
初孫は目の中に入れても痛くないほど可愛い。
My first grandchild is so cute I could just eat them up.
彼女は飼い猫を目の中に入れても痛くないくらい愛している。
She loves her pet cat so much that she thinks the sun shines out of its eyes.
かわいい is a general adjective for 'cute' or 'lovable', while 目の中に入れても痛くない is a hyperbolic idiomatic expression for extreme doting affection.
The expression literally means 'it wouldn't hurt even if I put them in my eye', hyperbolically conveying that someone is so precious that even the most sensitive act would cause no pain. The exact origin is uncertain, but it is a long-established Japanese idiom.