目の中に入れても痛くない is such a long phrase but a great word to use in conversation! It literally means “it’s right in the middle of my eye but doesn’t hurt”, but this expression is really a metaphor of how dear another person or animal (usually something dependent on you) is to you. The closest English expression would be “the apple of my eye.”
When something like a bug or dust particle gets into your eye, it causes so much pain or irritation. However, for things you truly care about, your love towards him or her is so strong, you are able to overcome the pain. And the reason we use the word 目, or “eye”, as the thing receiving the pain it’s because 目の中に入れる (“to get into one’s eye”) can also have a second meaning of “to see or view something”.
We do not use 目の中に入れても痛くない to express a romantic love or admiration of a person, such as your partner, friend, boss, etc. We usually use 目の中に入れても痛くない for someone who we take care of or who needs our care, like a baby, a child, student, or animal - something that needs our help (and sometimes is also really cute!)
目の中に入れても痛くない works as normal い-adjective. However, we can’t really change the phrase to the positive form, like 目の中に入れても痛い (“it’s painful if it gets into my eye), and create the opposite meaning of “I don’t love the person.” It is usually just used as-is.
For grandparents, grandkids are the apples of their eyes.
Puppies are so super duper cute!