An idiom meaning to love someone so much that you are blind to their faults; to dote on someone.
母親は息子のことを目に入れても痛くないほど可愛がっている。
The mother dotes on her son so much that she is blind to his faults.
あの人は猫を目に入れても痛くないみたいで、毎日写真を撮っている。
He seems to dote on his cat — he takes pictures of it every day.
自分の子供に対しては、つい目に入れても痛くないと思って甘くなることがある。
With one's own children, one can sometimes become indulgent and feel as if they are the apple of one's eye.