ANSWER:

The marriage is not done according to the sunnah and once they know, they must rectify it and correct it.

However, the children are considered to be legitimate for their parents as their marriage was done in the wrong but they were not living in zina. 

Sheikh, the way many marriages take place in our country are against the sunnah, and in one of ur post u said that the marriage is not valid. So will the child be able to get the wealth of the father even if they dont follow the sunnah in marriage? I mean, in the society they think this marriage to be valid.

Fiqh of Marriage, Marriage Contract | September 9, 2021 11:01 am
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In ind/pak the father gives his consent to marriage,is present during the marriage but doesnt say “i give u my daughter in marriage”. The guy and the girl say we accept the marriage in front of 2 witnesses and then the witnesses sign on the nikah contract. I saw ur video related to it but it wasnt clear. So is the nikkah valid? As from years this procedure is being followed
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If a persons wedding was arranged by parents where there was free mixing. Can this be regarded as kufr? Some scholars quoted Nisa 4:140 and Luqman 31:6 from Quran as saying Allah’s verses are being denied and mocked. Because if groom and bride commited kufr by attending this gathering before consumating then it means that marriage is annulled?