Is a prisoner's threat to kill a daughter if one prays considered coercion that permits abandoning prayer?

If the person threatening to kill the child is capable of doing so, then the threat is considered a valid form of duress. If the individual is unable to prevent harm from coming to his daughter, and the one threatening is capable of carrying out the killing, then the individual may pray inwardly without demonstrating the physical movements of prayer; because the one imposing duress cannot know what is in the heart, and the one under duress is absolved from what he is unable to perform of the prayer's actions. It has been mentioned that torturing a child is considered a form of duress.