Welcome back to the Wefugees Community and thank you for reaching out to us again.
I can not share much of experience on that topic but I tried to make some research. As far as I understand, the "Negativbescheinigung" in that context is needed to prove that you have the sole custody (so it is not needed for parents with shared custody as they most probably would have to sign an application for a passport together/both of them). I read that you can apply for it at the respective "Jugendamt".
Do you agree with my explanation dear @mbeon-fardeen or is there anything else you could add?
In any case, please don't hesitate to get back to us with any further questions.
All the best,