Here is how to add complex map to complex map.

add_map_to_complex_map.dart

void main() {
  Map parentMap = {
    'childOne': {
      'grandSon': {
        'grandGrandChild': {
          'childOne': 'John',
          'childTwo': 'Ron'
        }
      },
      'grandDaughter': {
        'grandGrandChild': {
          'childOne': 'Lilly',
          'childTwo': 'Sholin'
        },
      },
    },
    'childTwo': {
      'grandSon': {'childOne': 'Charly'}
    }
  };
  
  Map newMap = {
    'childOne': {
      'grandSon': {
        'grandGrandChild': {
          'childOne': 'John',
          'childTwo': 'Ron'
        }
      }
    }
  };
  
  Map tempParentMap = Map.from(parentMap);
  tempParentMap.forEach((key, value) {
    if (newMap.containsKey(key)) {
      parentMap.addAll(newMap[key]);
    }
  });
  print(parentMap);
}