Shaykh Sulaiman ar-Ruhayli said in the explanation of al-Wasiyyah as-Sughraa of Ibn Taymiyyah page 131:
"And from the greatest of those things which benefit is the dua (supplication) of the child for his parents; especially the righteous, for indeed it has been authenticated
(in a narration) that a mans level will be raised in paradise and he will say:
"How is this? How did I reach this? He knows that he is not from those deserving this level. So it will be said;
By the seeking forgiveness of your child for you."
Authenticated by al-Albani in his Silsilah as-Saheehah
So the righteous child should continue seeking forgiveness for his parents so that they will be forgiven and then their level will be raised in paradise.
(The child should seek forgiveness for his parent with any su
pplication which entails forgiveness for them)
Translated by Abu Anas Atif Hasan