code: def find_children(self):
          
          for i in range(len(self.vertices)):
              self.vertices[i].children = []
          for i in range(len(self.vertices)):
              for parent in self.vertices[i].parents:
                  if i not in self.vertices[parent].children:
                      self.vertices[parent].children.append(i)
docs: Take a tree and set the children according to the parents.
  
          Takes a tree structure which lists the parents of each vertex
          and computes the children for each vertex and places them in.