Does the reward for reminding people of abandoned Sunnahs reach the deceased, if done through a page created by his children to pray for him?

There is no objection to creating a Facebook page to pray for the deceased, because praying for the deceased is a great act that benefits them. The Prophet, peace and blessings be upon him, said: "When a person dies, all his deeds cease except for three: a continuous charity, knowledge from which benefit is derived, or a righteous child who prays for him."

Reminding people of forgotten Sunnahs is a good deed, and its doer hopes for its reward and the reward of those who act upon his reminder. As for dedicating the reward of good deeds to the deceased, this is a matter of scholarly disagreement. The Hanafis and Hanbalis held the view that the reward of such deeds reaches the deceased. Based on this opinion, there is no harm for the page creator to dedicate the reward of what comes from it to the deceased, or for one who offers an admonition or reminder to dedicate its reward to him. Similarly, if Qur'an, Hadith, or a lesson in knowledge is posted on it, and the poster dedicates its reward to the deceased, it will benefit him, Allah willing.