Is expiation obligatory upon a mother whose infant died of suffocation from a blanket after she had left him sleeping beside an opening in the bed that she had blocked with a light blanket?

If the child died by suffocation from the cover without any action on your part, then there is no blame upon you. And if there is doubt about the cause of death, then expiation is not obligatory, because the fundamental principle is the acquittal of one’s responsibility, and no penalty or fasting is obligatory except with certainty. So if you did not place the cover over the child’s face, but rather it became entangled by his movement, then there is nothing upon either of you. However, if you placed the cover over his face, then each of you owes the expiation for accidental killing, which is freeing a believing slave. If you cannot find one, then it is fasting for two consecutive months, along with repentance.