what happened?

so i recently redeemed 90 dollars of iTunes gift card credit, and i bought 100 dollars of an in app purchase, so i only would have to pay for ten dollars. I bought it, and my dad says he was charged 100 dollars of an ingame purchase. I checked MY iTunes credit(my family is using the family sharing), and it says account balance is ZERO dollars. iTunes says that you can use credit for in app purchases. why was i charged 100 dollars of purchases on iTunes credit and my dads credit card?!! PLEASE HELP! my dad is ****** at me.