The Offspring of Righteous Believers will be elevated to Their Grades in Paradise
In this Ayah, Allah the Exalted affirms His favor, generosity, graciousness, compassion and beneficence towards His creation. When the offspring of the righteous believers imitate their parents regarding faith, Allah will elevate the latter to the ranks of the former, even though the latter did not perform deeds as goodly as their parents. Allah will comfort the eyes of the parents by seeing their offspring elevated to their grades. Surely, Allah will gather them together in the best manner, and He will not decrease the reward or the grades of those higher in rank for joining them together, hence His statement,
The Believing Children of the Righteous will benefit from their relationship with their Believing Parents in Paradise
وَالَّذِينَ آمَنُوا وَاتَّبَعَتْهُمْ ذُرِّيَّتُهُم بِإِيمَانٍ أَلْحَقْنَا بِهِمْ ذُرِّيَّتَهُمْ (And those who believed and their children followed them in belief, We will join their children with them,... 52:21) Sayyidna Ibn 'Abbas ؓ reports from the Holy Prophet ﷺ who said:
"Verily, Allah elevates the ranks of the believers' children to the rank of their parents, even though the latter do not deserve the same rank as their parents deserve, so that the eyes of the parents are comforted." (Hakim, al-Baihaqi in his Sunan, al-Bazzar, Abu Nu'aim in al-Hilyah, ibn-Jarir and ibn-Abi Haim as quoted by Mazhari [ Tr.])
In the Hereafter, it will not happen that one man’s sins are attributed to another, and nobody will be able to enter Paradise except on the basis of his faith and his own righteous deeds. But, the occupants of Paradise will be given one special privilege: if parents are in a high stratum of Paradise and their children in some other stratum, the children will be united with their parents so that their happiness should increase. Only one who, in spite of being among his wife and children, has always been moved by the fear of God and who linked his hopes and fears with the one and only God, will be entitled to enter the sublime world of Paradise.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.