Accidental In-App Purchase - Howto cancel ?

Hello Everyone...

my nephew was over for a visit and he started playing with my ipad. Since this is not happening regularly I didn't activate restrictions on this device. After I downloaded this game for him the password remained active. Within this period he managed to perform an In-app purchase (he cannot even read) of $99.90.

I failed to contact the app developer. I failed to get apple support (issue cannot be helped at the current moment, try again later)

Does anybody know if there is a way to cancel this purchase other than blocking the transaction on my credit card?

Help is greatly appreciated! Thanks!