The warning is display after making an in-app purchase for the first time.
In-App Purchase Complete
You can now make additional purchases in any app for the next 15 minutes without reentering your password. To change this, tap Settings and go to Restrictions.
You can go to Settings:General:Restrictions to remove the 15 minutes window and require the immediate reenter of a password for future purchases.
The warning is thought to be a part of Apple's settlement with the FTC which has them refunding more than 37,000 in-app purchases that parents have claimed their children have made -- paying a minimum of $32.5 million.