This feature was just rolled out recently, from what I understand. You have to be on a desktop or laptop to access it.

Go to Settings and then Parental Controls. You'll be able to restrict shows by rating (Young Kids, Older Kids, etc.) and set a PIN to access those shows.

Below that, you can search for specific movies or shows and require the PIN to access them. For example, all the Barbie movies are normally allowed under the Young Kids category. But now, if my DD tries to access them, it will require a PIN.

BARBIE'S REIGN IS OVER!

I only wish you could hide the movie or show as well, so it wouldn't show up in the suggestion reel. But at least this is something!