This app was removed from the App Store.
Mac OS Video
Lock down the usage of YouTube on your Mac. Only allow videos that match criteria you consider safe for your children, under an application password you choose. Prevent "related videos" from appearing. Allow your child to save videos they like from the search criteria for later viewing.
New video player. Much faster. This has on screen buttons for fullscreen, play/pause, and a bar to let you drag to any place within the video.