Do you have arbitrators in Germany?
In the states, they are not lawyers, but serve in a law capacity, so to speak. They sit in on families (and other relationships, such as in business, to be the mediator when problems seem unsolvable.
These guys need to sit down and talk this through, with a mediator who knows family law. Also, a child advocate should sit in on the meetings. They could make these two knuckleheads realize what they might be doing to their own flesh and blood.