A Callous is just skin that has lost its sensitivity. If you lose your callouses you will have tender feet or hands. Sin makes us spiritually insensitive. Our hearts are hardened. We do not respond to God.
Children are sensitive.
I will never forget the strong feelings that came over me one day when I was a boy standing in our front yard. I had in my hand a new slingshot. Over my head towered the largest Elm tree in our little town. On a limb about 30 feet above me was a bird. He was singing.