Did you follow the step by step instructions in the Apple Support article I linked below?
Set up and use Apple Cash Family - Apple Support
You’ll need to setup Face ID and Two Factor Authentication (2FA), if not already done so.
Set up Apple Cash for a child or teen
On the family organizer's iPhone, open the Settings app.
Tap [your name], then tap Family Sharing.
Tap the child or teen that you want to set up Apple Cash for.
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.
Set up and use Apple Cash Family - Apple Support