Welcome back to the Wefugees Community and thank you for reaching out to us.
From my experience, the authorities do ask people who married a German citizen in Germany, but originally came on another type of Visa (Schengen visa for instance) to travel back and apply for the spouse visa at the respective German embassy. I haven't heard of a case in which applicants for a residence permit according to §28 of the German residence law (as parent of an underaged German child) were asked to do the same.
However, I will also link two of our experts here. Dear @mbeon-Christine or @alla_fka, am I right on this or is there anything you may want to add?
I also checked the website of the German embassy in Nigeria. Apparently, due to the Corona situation they don't accept any visa applications at the moment.
Please don't hesitate to get back to us with any further questions.
All the best,