Do you let you children or others use your iPhone or iPad? It can be like handing them your credit card when it comes to in-app purchases.
How to set purchase restrictions:
You can adjust settings to require a passcode for every purchase to avoid surprise purchases.
1) Go to Settings>General>Restrictions and tap Enable Restrictions.
2) Choose and enter a restrictions passcode and then enter it again. (This should be different than the passcode your child uses to access her iPhone/iPad.)
4) Scroll down and tap on Require Password and select Immediately. You now have control.
To disable all in-app purchases, go back to the Restriction settings (in General Settings) and toggle In-App Purchases off.