What is the ruling of the Shari'ah concerning a wife who strongly refuses to allow her son, who is not yet three and a half years old, to see his grandmother (the husband's mother)?

A wife preventing her son from visiting his grandmother is considered a severing of kinship ties (qati'at ar-rahim). The husband must admonish her and prevent her from doing so, informing her that she is sinful and exposed to Allah Almighty's punishment. He should not accede to her request. Rather, he must take his son to his mother (the son's grandmother) whenever he wishes.