def get_child_schema(self):
    """An optional function which returns the list of child keys that are associated
with the parent key `docs` defined in `self.schema`.

This API returns an array of JSON objects, with the possible fields shown in the example.
Hence the return type is list of lists, because this plugin returns
a list of objects, each with this possible set of keys.

Returns:

    [['year', 'title', 'description', 'mediatype', 'publicdate', 'downloads', 'week', 
    'month', 'identifier', 'format', 'collection', 'creator', 'score']]

Example of one of the child objects in the array associated with `docs`:

    {
        year: 1998,
        title: "AAPL CONTROL ROOM AERO ACOUSTIC PROPULSION LABORATORY AND CONTROL ROOM PERSONNEL",
        description: "AAPL CONTROL ROOM AERO ACOUSTIC PROPULSION LABORATORY AND CONTROL ROOM PERSONNEL",
        mediatype: "image",
        publicdate: "2009-09-17T17:14:53Z",
        downloads: 5,
        week: 0,
        month: 0,
        identifier: "GRC-C-1998-853",
        format: [
            "JPEG",
            "JPEG Thumb",
            "Metadata"
        ],
        collection: [
            "nasa",
            "glennresearchcentercollection"
        ],
        creator: [
            "NASA/Glenn Research Center"
        ],
        score: 2.617863
    }
"""
    return [['year', 'title', 'description', 'mediatype', 'publicdate', 'downloads', 'week',
             'month', 'identifier', 'format', 'collection', 'creator', 'score']]