Please contact Apple Support. You’ll need to contact both Apple Support and the transit companies support. Both of those entities will have confronted this issue before. We can continue troubleshooting the issue here, but it’ll be mostly basics.
Is your child logged into their Apple Account? Do they have more than one Apple Account ( Apple ID)? Many children create multiple accounts to bypass their parents or guardians supervision.
Have you confirmed with both Apple Support and the support team at the transit company the minimum age requirements?
Is your child’s device updated to the latest iOS version, 18.5? If not what version are they using?
Or you can contact Apple Support. 😀😀😀