Dear @Eli83,
This is not an area I am specialised in but generally in such cases, where a child is resident in Germany and an agreement cannot be made between the parents, a family court will decide upon the matter. The court would assess each case individually, with the welfare of the child the deciding factor on where they should reside in the future and how the other parent can continue to exercise their access rights.
I recommend you **** at the information provided by this organisation:
https://zank.de/en/
They are specialised in this field. They also have information in Russian on their site.
Throughout German there are youth welfare offices (Jugendamt) and family advisory services. Such offices are also there to assist both parents and children in such situations. Here you can search the directory for such assistance:
https://zank.de/en/materials-addresses/contacts/
The German Federal Office of Justice also provuides information in English about international custody matters and their role in such cases:
https://***.bundesjustizamt.de/EN/Topics/FamilyMattersInternational/Custody/Custody_node.html
I hope that helps, let us know if you have any further questions.
Best,
Éanna