[The Ruling on a Child Visiting the Grave of His Immoral Father]

Answer – and Allah is the Guardian who grants success: There are two matters here, namely: the visit, and the supplication (duʿā’).
As for supplicating for the immoral father for mercy, forgiveness, and the reward of the Hereafter, that is not permissible, due to His saying, the Exalted: “It is not for the Prophet or the believers to seek forgiveness for the polytheists, even if they are close relatives, after it has become clear to them that they are the people of the Hellfire. And Abraham’s prayer for his father was only because of a promise he had made to him; but when it became clear to him that he was an enemy to Allah, he disassociated himself from him. Indeed Abraham was truly tender-hearted and forbearing.” [Al-Tawbah:113–114]
As for the mere visit that is free of supplication for the father, it is permissible for a purpose, such as if he fears people’s blame if he were to leave it, or that they would say: “Look at this religious man—see how his religiosity leads him to abandon visiting his own father,” which would result in driving people away from listening to him and accepting what comes from him.
Furthermore, there is no doubt about the permissibility of visiting a disbelieving or immoral father during his lifetime, due to His saying, the Exalted: “…and keep their company in this world in a good manner…” [Luqmān:15] and His saying: “…and be good to parents…” [Al-Baqarah: 83] So when the father dies, the basic ruling is the permissibility of visiting him, by continuing the first ruling, and nothing has appeared to us that would change this basis; the verse only forbids seeking forgiveness for the enemies of Allah.
As for the ḥadīth in which the Prophet, May Allah bless him and his family and grant them peace, said: “I asked my Lord for permission to visit my mother, and He did not grant me permission except for an hour…,” this is the meaning of the ḥadīth. Perhaps that was from the Prophet, May Allah bless him and his family and grant them peace, in the early period, for he (May Allah bless him and his family and grant them peace) had previously forbidden visiting graves, then after that he commanded it, saying: “Indeed, visit them, for they remind one of the Hereafter.” This ḥadīth is among the solitary reports (āḥād) according to Ahl al-Sunnah, and it has no existence in the narrations of the Zaydīs.
Visiting graves for admonition and reflection is permissible absolutely, whether the people in the graves are Muslims or non-Muslims. This may be indicated by the saying of Allah, Exalted is He, regarding Pharaoh: “So today We will save you in your body so that you may be a sign for those who come after you.” [Yūnus:92] What appears to me to be forbidden is that the motive for visiting the grave be that the deceased was an enemy of Allah; if that is the motive for visiting him, then the visit is not permissible.
And it is permissible to visit the grave of an immoral or unjust person with the intention of taking admonition, or to supplicate against him. It has been related that Imam ʿIzz al-Dīn ibn al-Ḥasan, Peace be upon him, when he stood at the grave of Nashwān al-Ḥimyari, recited lines of poetry in which he mentioned his rank in knowledge, were it not for nasb [hostility towards the Prophet’s family, Peace be upon them].
As for visiting graves in order to seek reward and recompense from Allah, this is only in regard to the righteous servants of Allah, for visiting them contains reward and recompense. The Prophet, May Allah bless him and his family and grant them peace, used to visit the people of al-Baqīʿ, stand there for a long time, and supplicate for them; and he visited the martyrs of Uḥud eight years after they were killed, and he prayed over them a prayer like his prayer over the funeral. Fāṭimah, Peace be upon her, used to visit the grave of her uncle Ḥamzah ibn ʿAbd al-Muṭṭalib at Uḥud every week. And the Muslims, in all eras, have continued to visit the grave of the Prophet, May Allah bless him and his family and grant them peace, and the graves of the martyrs of Uḥud and the people of al-Baqīʿ; and all the Islamic schools of law, both Shīʿah and Sunnī, seek nearness to Allah by visiting the Prophet, May Allah bless him and his family and grant them peace, the people of Uḥud, the people of al-Baqīʿ, and the graves of the Imams, scholars, and devoted worshippers.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2