Use get_child_descriptors for grades.py

lms/djangoapps/courseware/grades.py

@@ -36,8 +36,7 @@ def yield_dynamic_descriptor_descendents(descriptor, module_creator):
     def get_dynamic_descriptor_children(descriptor):
         if descriptor.has_dynamic_children():
             module = module_creator(descriptor)
-            child_locations = module.get_children_locations()
-            return [descriptor.system.load_item(child_location) for child_location in child_locations ]
+            return module.get_child_descriptors()
         else:
             return descriptor.get_children()