You don't provide any details such as where you are seeing this subscription. Is the nephew part of Family Sharing? If yes, then how old? If not, then are you seeing the subscriptions on your account or just charges on a credit card? Did you nephew have access to your device? Etc. Knowing that kind of information would help you to avoid having to read all the documents about the various applicable scenarios.
Please click on this link to learn "How to cancel a subscription from Apple" ➔ https://support.apple.com/HT202039
Note:
• "If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew." An "Expires" date (instead of "Renews") means the current subscription will just run until then. "Resubscribe" is just in case you change your mind about having it end.
• Deleting an app does not cancel a subscription.
• If you don’t cancel trials more than 24 hours before your renewal date, your subscription might renew.
• If you don't have an Apple device, see the options near the very bottom of the support document.
"If the subscription that you want to cancel is not listed", click here ➔ https://support.apple.com/HT212052
If you still can't find a subscription, contact Apple Support using the link at the end of the support article.
If you still have problems:
Click here ➔ https://support.apple.com/choose-country-region/media-services Select your country, then a product.
Or: Contact Apple for support and service ➔ "See a list of Apple phone numbers around the world."
Click here ➔ https://support.apple.com/HT201232
If you are having problems with unauthorized purchases being made on your device you may wish to always 'Require a password for App Store and iTunes purchases'.
Click here ➔ https://support.apple.com/HT204030
"You can use Screen Time to prevent unintentional or unauthorized purchases from the App Store and other Apple services. You can require a password for purchases, prevent certain types of purchases, or disable purchasing completely." - https://support.apple.com/HT204396 See how to 'Use Screen Time on an iPhone, iPad, or iPod touch' by clicking here --> https://support.apple.com/HT208982