Either Google flat-out never mentioned this, or it's badly slipped our memory, but Google's new family plans aren't just for Google Play Music. Once you set up your family plan account you'll get an email with more information. And in that email is a link to Google's support docs for all this, and in that you get two clear lists of what you can and can't pay for with a family payment method.
With a family payment method you can buy apps, books, movies, games, TV shows and magazine issues. Not available are devices from the Google Store, Google Play balance, and Android Pay purchases.
Also note that by default everyone in your family will have to get approval to make in-app purchases. But you turn that off for family members individually — at your own peril.
Source: Google support