See -> Request a refund for apps or content that you bought from Apple.
Request a refund for apps or content you've bought from Apple – Apple Support (UK)
If you have requested a refund, you must wait until Apple has approved. Then you will receive a message from Apple.
Refunds are not guaranteed. For more information, see the Terms & Conditions.
For the future:
Manage App Store purchases, subscriptions, settings, and restrictions on iPhone
Manage App Store purchases, subscriptions, settings, and restrictions on iPhone – Apple Support (UK)
Prevent in-app purchases from the App Store
Prevent in-app purchases from the App Store – Apple Support (UK)
You can use Screen Time to prevent unintentional or unauthorised purchases from the App Store and other Apple services. You can make a password a requirement for purchases, prevent certain types of purchase, or disable purchasing completely.