What is the ruling on frequently saying "May Allah, for whose sake you have loved me, love you" in response to a son's declaration of love, and does the ruling and moral perspective on this differ between a son and a daughter?

A son's love for his father is a blessing, and expressing this love is neither shameful nor problematic. The Prophet, peace and blessings be upon him, encouraged this, as stated in the hadiths, among them his saying: "If a man loves his brother, let him inform him that he loves him," and his saying to Mu'adh: "By Allah, I love you." It is incumbent to reciprocate the son's love and to draw closer to him, raising him with seriousness by involving him in decisions and entrusting him with responsibility, along with guidance and advice, for he who shows no mercy will not be shown mercy. Moreover, showing affection does not contradict seriousness and steadfastness in facing life; rather, seriousness is measured by one's general conduct, good behavior, and ability to confront problems.