'Creates and saves a new xblock that as a child of the specified block
Returns the newly created item.
Args:
user_id: ID of the user creating and saving the xmodule
parent_usage_key: a :class:`~opaque_key.edx.UsageKey` identifing the
block that this item should be parented under
block_type: The type of block to create
block_id: a unique identifier for the new item. If not supplied,
a new identifier will be generated
fields (dict): A dictionary specifying initial values for some or all fields
in the newly created block'

def create_child(self, user_id, parent_usage_key, block_type, block_id=None, fields=None, **kwargs):

    item = self.create_item(user_id, parent_usage_key.course_key, block_type, block_id=block_id, fields=fields, **kwargs)
    parent = self.get_item(parent_usage_key)
    parent.children.append(item.location)
    self.update_item(parent, user_id)
