code: def get_child_classes(cls, slot, page, instance=None):
          
          child_classes = super(BootstrapCardPlugin, cls).get_child_classes(slot, page, instance)
          # allow only one child of type Header, Body, Footer
          for child in instance.get_children():
              if child.plugin_type in child_classes:
                  child_classes.remove(child.plugin_type)
          return child_classes
docs: Restrict child classes of Card to one of each: Header, Body and Footer