1) Open the Store app.
2) Tap the three dots icon on the bottom-right and tap Settings.
3) Tap PIN.
4) Tap the button next to Wallet PIN to turn it On.
5) Enter a 4-digit PIN twice and tap done.
6) Click Use Wallet PIN to protect music, app and in-app purchases.
Now in-app purchases won't be completed unless the wallet PIN is entered.