Hello Rita manu,
I think Meike is right in supposing that the reason for giving you a weaker residence permit after §25.5 (impossibility of deportation), instead of §126.96.36.199 (German child), can be coming to Germany illegally. More exactly you can only know if you ask your Ausländerbehörde for an explanation.
In such cases the Ausländerbehörde can make a claim that you go to your country and apply for a proper visa (for family reunion to your German child). Sometimes it can be really a good way to solve the problem - if you can afford it. The whole preparation process of applying for such a visa can be made from Germany, so that you just travel for an appointment with the German embassy and get your visa in a short time. For that you need a confirmation from the Ausländerbehörde in advance that they agree to issuing this visa for you (so called Vorabzustimmung).
I know that in most cases this way is unfortunately difficult or impossible. And some Ausländerbehörden are very strict and don't agree to give you better than §25.5. For the question if it is worth making a legal process you must consult a good lawyer.
The father of your future child must make an acknowledgement of fathership and a custody of a child, if he hopes to stay because of the child. The problem is that if both parents have custody but only one parent has a residence permit, it is in discretionary power of Ausländerbehörde to give the child a residence permit after §33. They can also give the residence after §25.5 or even first only Duldung, especially if the living costs are not covered from the own income of the parents.
If the child gets a residence permit, it would be possible to try it for the father, but it can be difficult and it probably will be impossible if the father doesn't earn enough for all costs, doesn't have a passport, has criminal penalties etc. As the child won't be German, the father isn't fully safe, especially if he doesn't live together with the child.
I don't think it is possible to get more exact information online. It is important to know how such cases are handled in your place. Therefore you should ask for help from your local councelling or a lawyer.
If your new