Apple Pay not working on a 13 years old child's iPhone.

Hello Borubar111,

Thank you for reaching out to Apple Support Communities.

These steps may be able to help:

"Start here

The family organizer that set up Family Sharing for your family group must use Apple Cash Family to set up Apple Cash for family members under the age of 18.

If you're the family organizer:

• When you set up your Apple Cash account, use the same Apple ID that you used to set up Family Sharing.
• At least one member of your family group must be under 18 years old.

Requirements for the organizer and members of the family group

• Own a compatible iPhone, iPad, or Apple Watch with the latest version of iOS, iPadOS, or watchOS.
• Sign in to iCloud on each family member's device using their Apple ID
• Turn on two-factor authentication for each Apple ID."

More detail scan be found here: Set up and use Apple Cash Family

Keep in mind that different countries may have different age requirements to use Apple Pay and Apple Cash.

Can you provide the exact translation of the alerts you are seeing? Also, Are you attempting to set up Apple Cash? If not, what kind of card are you using with Apple Pay?

Cheers!