There are several ways you can try to resolve this issue:
Contact a ticket agent: You can find a ticket agent in a nearby city. Some ticket agents may be able to help handle information authentication issues.
Consult the train station: You can consult directly at the nearby train station to see if there are other solutions, or to find out whether you can entrust others to handle it for you.
Contact Customer Service: You can call the railway customer service hotline 12306 to explain the situation in detail and seek official help and advice.
Try online refund: If the refund system supports it, you can try online operation to see if the authentication can be completed through other methods.
Seek help from family members: If the child is unable to certify in person, you can try to have the child’s immediate family member or legal guardian certify on your behalf.
Mail proof: If it is indeed impossible to authenticate on site, you can try to mail relevant certification materials to the train station or railway customer service center to request review.
Hope these suggestions are helpful to you.