I subscribed for my son... Now his phone is broke and he can't get into it how do I cancel this subscription?

Payments are usually handled through the app store account associated with the device. iPhone app store, google play etc. Log into the account that was on the broken phone on any other device and you should be able to access the subscriptions for that account. That is where you cancel. If you have trouble then you will need to find a support option to contact the app store and get help with cancelling.

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