A spiritual aspirant wrote,

On the death of (my) child his mother is (extremely) disturbed. She cries repeatedly on simple matters and expresses sorrow. Compared to her despair my grief is nothing. Moreover, on hearing from people about losing a child I feel minimal sorrow in my heart. Because of this I fear that my heart has hardened (قساوت القلب) and I have no attachment with my children.

Hakim al Umma Maulana Ashraf Ali Thanvi (Allah have mercy on him) replied,

‘Having attachment (to children) in itself is not a requirement. Therefore, its deficiency or absence should not be worrisome. Attachment is required to fulfill their rights. There should not be any deficiency in this.

Hardening of heart entails being brave on committing sinful.

Lack of attachment and feelings is not hardening of heart. It is something that is, also, to an extent coveted.

Tarbiyet us Salik, volume 3, p174-5