Is it probable that the grandmother caused the infant's death due to the presence of drops of milk in her mouth? What is the ruling in this case after the grandmother's death? And what is the ruling if the mother (who was 3 years old at the time) was the one who caused the infant's death?

Accidental killing necessitates blood money (Diyah) and expiation (Kaffarah). The blood money is incumbent upon the 'Aqilah (male relatives on the paternal side) of the person who caused the killing. As for expiation, it is the freeing of a believing slave; if one cannot be found, then fasting for two consecutive months. The expiation is obligatory from the killer's wealth, even if the killer is a child or insane, and this is the view of the majority of scholars. If it is not proven that either of them caused the death of the infant, then nothing is obligatory upon them.