Why has my refund not been fully refunded i knly got £20 out of the £30 i refunded because my son brought it without my permission

Accidental purchase?

1. App Purchase
   1. Curious about a Charge? --> If you see an 'itunes.com/bill' or an unfamiliar charge on your statement
   2. If you have an issue with an app on App Store or billing --> Request a refund for an App Store or iTunes Store
2. Subscriptions
   1. You may view a subscription to an app --> See or cancel your subscriptions - Apple Support
   2. You can also request a refund for a recent renewal of an in-app subscription purchase 

Avoid accidental purchases

1. If it is your device then, please --> Use Guided Access with iPhone, iPad, and iPod touch - Apple ... before handing over a device to a child.
2. If it is your child's device then in the future, enable the Ask to Buy feature --> Request and make purchases with Ask to Buy - Apple Support

It could be Phishing

• Beware of --> Phishing & Other Suspicious Emails 
• Know how to --> Identify legitimate emails from the App Store or iTunes Store 
• Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams —> Use these tips to avoid scams and learn what to do if you think your Apple ID has been compromised.