Answer
(Fatwa: 753/753/M=1432)
Why do you want to marry a Hindu girl? Are there no Muslim girls in the society? It is not lawful to marry a Hindu girl.
لاَ تَنكِحُواْ الْمُشْرِكَاتِ حَتَّى يُؤْمِنَّ [البقرة : 221]
Marriage done without accepting Islam shall be invalid, so how the children born out of it can be legitimate? Likewise, Islam accepted merely for marriage cannot be trusted. It is feared lest you should also not lose your Islam. You are sincerely advised to give up the intention of marrying that Hindu girl and find out any suitable Muslim girl for you.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband