'Adds a parent to child relationship in the given block
relations map.
Arguments:
block_relations (dict({UsageKey: _BlockRelations})) -
Internal map of a block\'s usage key to its
parents/children relations.
parent_key (UsageKey) - Usage key of the parent block.
child_key (UsageKey) - Usage key of the child block.'

@staticmethod
def _add_to_relations(block_relations, parent_key, child_key):

    BlockStructure._add_block(block_relations, parent_key)
    BlockStructure._add_block(block_relations, child_key)
    block_relations[child_key].parents.append(parent_key)
    block_relations[parent_key].children.append(child_key)
