Request for refund
Why am I not eligible for a refund on an accidental purchase that my child made!?
iPhone 7, iOS 12
All purchases are considered final as you know, any refunds are at iTunes Support's discretion. You could try contacting them again : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)