Is it permissible to postpone the fasting for the expiation of my son’s death – who rolled over in bed and passed away – until autumn and winter, knowing that I am a nursing mother and that this will exceed three years from his death?

If the mother is the cause of her infant's death, then the rulings of accidental killing apply to her. However, if the infant dies without her involvement, or if she is unsure, then there is no blood money (diyah) or expiation (kaffarah) upon her. Ibn Hazm is of the opinion that if the mother accidentally causes the death of her child, she is liable for expiation, and her 'aqilah (male relatives on the father's side) are liable for the blood money. If she is unsure, then there is no blood money or expiation. The fasting for expiation can be performed at leisure, with no disagreement among scholars. As for the three-year period, it pertains to the deferment of blood money upon the 'aqilah, not the expiation.