All countries of the European Union acknowledge the right of a child to habe personal relationship a personal contact to both parents even if they live in different countries. Advice and support on the question of whether and how contact with the parent living abroad can be given by the youth welfare offices or, in the case of conflict, the family court.
Youth welfare offices in Hamburg:
http://www.hamburg.de/vor-dem-verfahren/3765220/jugendaemter/