This script will create a collection of your movies based on the movie rating, ie. If you want all G rated movies in a collection on it's own.

plex_create_rating_collection.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# Description:       Create a Plex collection from based on content rating.
# Original Author:   /u/SwiftPanda16
# Modified by:       @bordignon on Twitter.
# Requires:          plexapi, requests
#

import requests
from plexapi.server import PlexServer


### EDIT SETTINGS ###

PLEX_URL = 'http://localhost:32400'
PLEX_TOKEN =  'xxxxxxxxxxxx'

PLEX_LIBRARY_NAME = 'Movies' # where the movie's are located.

COLLECTION_NAME = 'Kids'  # New collection name, ie. KIDS movies.

CONTENT_RATING_SEARCH = ['G','PG']  # The Movie's Content Rating, ie. G or PG for kids movies.

### CODE BELOW ###

def add_collection(media_item):
    library_key = media_item.librarySectionID
    rating_key = media_item.ratingKey
    collection_key = "collection[{key}].tag.tag".format(key=len(media_item.collections))

    headers = {"X-Plex-Token": PLEX_TOKEN}
    params = {"type": 1,
              "id": rating_key,
              "collection.locked": 1,
              collection_key: COLLECTION_NAME
              }

    url = "{base_url}/library/sections/{library}/all".format(base_url=PLEX_URL, library=library_key)
    r = requests.put(url, headers=headers, params=params)
    

def main():
    try:
        plex = PlexServer(PLEX_URL, PLEX_TOKEN)
    except:
        print("No Plex server found at '{base_url}', or invalid token.".format(base_url=PLEX_URL))
        print("Exiting script.")
        return

    movies = plex.library.section(PLEX_LIBRARY_NAME)

    x = 0
    y = 0


    #for video in movies.search(unwatched=True):
    for key in CONTENT_RATING_SEARCH:
        
            for video in movies.search(contentRating=key): 
                
                print("%s (ID:%s) Rating: %s") % (video.title, video.ratingKey, video.contentRating)
                x = x + 1

                try: 
                    media_item = plex.library.getByKey(video.ratingKey)

                    if COLLECTION_NAME.lower() not in [c.tag.lower() for c in media_item.collections]:
                        add_collection(media_item)
                        y = y + 1
                    else:
                        print("   This movie already exists in this Collection. Skipping.")
                        continue

                except:
                    print("Error adding")
                    pass


    print("Movie's found with matching rating: %s" % (x))
    print("Movie's added to collection: %s" % (y))




if __name__ == "__main__":
    main()