Welcome to iTunes Store Customer Support. My name is Rahul and I am glad to assist you.
I understand that your son purchased some items while playing with an application. I realize that you don't want to be charged for content that was not purchased intentionally. I'll be happy to make this right for you.
Lee, I checked your account and it looks like the purchase "Chest of Gems" was purchased from within the application "Clash of Clans" on your iPhone, iPad, or iPod touch. This is called an "In-App Purchase". In App purchases are: an item that can be purchased within an application (that has also been purchased on your iTunes Account). For example, if you were to purchase an Army Game application, within that game you can also purchase guns, armor, etc., which would be In App content. For information on this type of purchase, check out this article:
I understand that the purchase of "Chest of Gems" was unintentional. In five to seven business days, a credit of 788.00 HKD, plus any applicable taxes, should be posted to the credit card that appears on the receipt for that purchase.
Please note that this is a one-time exception, as the iTunes Store Terms and Conditions state that all sales are final.