I got charged around over 700$ for games in one day App Store games without my permission

1.

If an app 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.

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)

3.

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

Apple is not here.

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.

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.

That your child is making charges without your permission is not a reason for a refund. It is up to you to set parental controls that prevent your child from doing it. There have been numerous examples of your situation in which refunds have been denied by Apple or the third party developer. There have also been examples of Apple agreeing to a refund. You will need to contact Apple to appeal your case.