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

If Apple has notified you that it is not eligible for refund, we can't help you, sorry, we are also users of Apple products & services like you. 

Some purchases are final.

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my children signed up for picart 399k's trial package by themselves,

To avoid the case in the future, see ⤵️

If other people have access to your iPhone or other Apple device, change the device passcode and never share it with others.

Change your Passcode -> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Change your Apple ID password -> Change your Apple ID password - Apple Support

• Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 
• Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

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)

if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.

Or call direct Vietnam -> 1800 1127