If you're worried about unauthorized purchases being made on your account, don't worry: you can enable security features in iOS to block purchases unless a password is entered first, or disable In-App Purchases entirely.
To keep In-App Purchases enabled but have a password prompt appear every time you make a purchase, first close the game and open the iOS settings menu.
1. Select iTunes & App Store >Password Settings. Please note that this option will not appear if you're using Touch ID, as in this case you'll be using Touch ID to authorize every purchase anyway.
2. Under Purchases and In-App Purchases, tap the Always Require option. You can also chose to require a password to be entered to download free apps with the toggle here.
It's also possible to disable purchases completely.
To do this, close the game then opening the iOS Settings menu and navigate to General > Restrictions. Tap Enable Restrictions, then chose a memorable PIN when prompted. Once you've enabled restrictions, find the In-App Purchases toggle, and turn it off.