The free trial may mean that if you do not agree to "install" (upgrade) it after 30 days it will no longer work.
Alternatively from what I can find out, the app itself is free, however it does come with this "inclusion" in the app -
iTunes word it this way:
Episode lets you purchase items within the game for real money. Please disable in-app purchases on your device if you do not want this feature to be accessible.
Google Play Store to my mind phrase it better: (the same app but for Android devices)
Episode is free to play, but you are able to purchase game items with real money. If you want to limit the ability to make in-app purchases, you may create a PIN in the Settings menu from within the Google Play Store.
Also the terms of service agreement that you accept when you install the app states that if you are under the age of 13 you cannot create an account (presumably to pay for any in app purchases), and if there is unauthorized use of the account by minors you will be liable for any use of your credit card.
Hopefully this is of some help.