def _get_child_node_by_title(parent_node, title):
    """
    Looks through `parent_node`s children to see if a topic|cluster|lesson with
    the given title exists and returns the child_node, else returns None.
    """
    child_node = None
    for existing_child in parent_node['children']:
        if existing_child['title'] == title:
            child_node = existing_child
    return child_node