Welcome back to the Wefugees Community and thank you for your interesting question.
I assume that the answer is depending on some relevant details as the type of the fictional certificate (Fiktionsbescheinigung): Was it issued according to § 81 Section 4 or Section 3 of the German Residence Act? The fictional certficate according to Section 4, for instance, is issued if the applicant holds a currently valid residence permit, the decision on the new application is, however, still pending. Please see more information on the difference between these "types" of the ficitonal certificate here. In this case, the in Germany born child may be issued a residence permit according to Section 33 of the German Residence Act. If the other parent is holding a residence permit as well, the issuing might happen automatically. In case the other parent is not holding a valid residence permit (but a "Duldung", for instance), you would need to apply for it at the immigration office.
Dear @mbe-on_Zsigó or @mbeon-fardeen, are my explanations correct and do you want to add anything?
Please also don't hesitate to get back to us with any further question.
All the best,