'Returns the children of the block identified by the given
usage_key.
Arguments:
usage_key - The usage key of the block whose children
are to be returned.
Returns:
[UsageKey] - A list of usage keys of the block\'s children.'

def get_children(self, usage_key):

    return (self._block_relations[usage_key].children if (usage_key in self) else [])
