Hi @dung nguyễn
Did the accidental purchase your child made through the Microsoft Store on Windows or Mobile? If it was through this, contact Microsoft Support via this link: https://support.microsoft.com/home/contact
If it was made through XBOX, you may contact XBOX Support via this link: https://support.xbox.com/en-US/help/subscriptions-billing/buy-games-apps/refund-orders
Hope they find a resolution!
Best regards,
Nanta