بسم الله الرحمن الرحيم
(Fatwa: 35/35/B=01/1437)
This must have been sent by any ghair Muqallid. It is against the Hadith. According to the Hadith qurbani is wajib on every rich (sahib nisab). Separate qurbani is wajib on both the husband and wife as well as separate qurbani is wajib on father and mother. If the children are adult and stay separately and are prosperous and are sahib nisab then each one will have to offer separate qurbani. Yes, if someone offers nafl qurbani and includes his family and the whole Muslim community in its rewards then it is alright.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband