Hi Lillipillipie
welcome to the Apple (user-to-user) Community.
Re: "Apple Wallet for Daughter aged 13 : It won’t let me add any cards to Apple wallet"
Seems Apple Pay works with the birth-date listed in a person's own Apple ID Account:
If the birthdate listed is correct, try updating the device to the newest available update,
or
try re-starting your daughter's device
( Eg: Restart your iPhone - Apple Support / Restart your iPad - Apple Support )
(ie: Restarting deletes temporary files, re-establishes connections, and allows for changes to update etc., and will often restore order if something is not working as expected.)
Else:
If birth-date is incorrect, see: Update the birth date associated with your Apple ID - Apple Support
If needed: You might want to reach out to Apple ID Support via their website online :
(Eg: online chat / phone / email etc):
There is a "Get Support - start now" link at the bottom of their webpage:
Apple ID - Official Apple Support
All the best :-) 🇦🇺