Request a refund

1.

my child subscribed to the app

Perhaps you must cancel the subscription before the next invoice arrives.

If the invoice has to do with an app that has a subscription associated with it, see -> Delete apps on your iPhone, iPad, and iPod touch -> Delete apps on your iPhone, iPad, and iPod touch - Apple Support

• If you delete an app, it doesn’t cancel in-app subscriptions that you might have. While deleting an app, you might see a Cancel Subscriptions option. Learn how to cancel your subscriptions.

If you don't want to continue a subscription you need always to cancel that subscription in time, see the following Apple articles:

If you want to cancel a subscription from Apple

If you want to cancel a subscription from Apple - Apple Support

If the subscription that you want to cancel is not listed

If the subscription that you want to cancel is not listed – Apple Support (UK)

2.

which I was refused.

Refunds are not guaranteed. For more information, see the Terms & Conditions.

We can't help you, sorry, we are also users of Apple products & services like you. 

3.

To avoid the case in the future, see:

Require a password for App Store and iTunes purchases

Require a password for App Store and iTunes purchases - Apple Support

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)

That is Apple's policy, that refunds are not guaranteed. For more information, see the Terms & Conditions.

We are also users of Apple products & services like you.

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

Hello ~ Take a look here for future help…

Require a password for App Store and iTunes purchases

~Katana-San~

Already posted in my 1st response.

Hello ~ Excuse me. I was just underlining a point.

~Katana-San~