Hello Tara.Wadlington,
You may need to check and ensure you have Apple Cash Family set up correctly from the organizer of the Family Sharing group before you can add Apple Cash to their account.
Set up and use Apple Cash Family outlines the process:
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.
* Make sure that each device region is set to the United States.
Once you have the family organizer account set up with Apple Cash, you should be able to add the teenager account from the device of the organizer if all the above criteria are met.
Set up Apple Cash for a child or teen
1. On the family organizer's iPhone, open the Settings app.
2. Tap [your name], then tap Family Sharing.
3. Tap the child or teen that you want to set up Apple Cash for.
4. Tap Apple Cash, then follow the instructions on your screen to set up their account. You might need to verify your identity.
If you see Pending, make sure that your child meets the requirements.
After you complete these steps, you can view and manage the Apple Cash accounts of your children and teens.
Cheers!