thank you for sharing your question with the Wefugees Community!
We discussed a similar situation earlier on this platform - you may want to check the following thread for more information. The most important documents you need to have are the proof of recognition of paternity (Vaterschaftsanerkennung) plus the certificate regarding legal custody. Please note that it is very much important to proof to the immigration office that you want to take care of your child and that you have been doing so in the past, for example by paying alimony and taking care of your child.
I hope this answer was useful for you. Please feel free to contact the community again if you need further assistance!
All the best,
Note: Please keep in mind that we can´t provide qualified legal advice via the internet and that any information given can´t replace such legal advice.