Subscribe or Unsubscribe to Amazon FreeTime Unlimited. From the Parent Settings screen of Amazon FreeTime, tap Manage Content & Subscription. Enter your parental controls password and tap OK. Under Manage Subscription Content, tap Subscribe to Amazon FreeTime Unlimited. Select a monthly subscription by step process of cancelling Amazon FreeTime Unlimited with screenshots
The directions as written didn't work - the options it said to select were not present. It did lead me close enough to figure it out myself, though. So if you have kindle unlimited, give it a shot. Even if it had worked exactly as intended, however, $2.99 would have been ridiculous for the contents.