subscription

You may cancel a subscription to an app --> See or cancel your subscriptions - Apple Support

You can also request a refund for a recent renewal of an in-app subscription purchase 

If you have purchased an app there is no way of getting the refund. However, you may try -->  Request a refund for an App Store or  iTunes Store purchase

At times subscription refund may not be possible, however you may change or cancel a subscription—or check if a charge from Apple that you don't recognize is for a subscription—tap or click this button:

• If the purchase is already made then I am afraid a refund is difficult. However you may try --> If you have an issue with an app on App Store or billing --> Request a refund for an App Store or iTunes Store
• 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.
• If it is your child's device then in future, enable ask to buy feature --> Request and make purchases with Ask to Buy - Apple Support