Apple Cash not available after setting it up.

I am having difficulty being able to send one of my minor children money through Apple Cash. I have twins. And just added them both to the Family Sharing through Apple. I then set both up with Apple Cash. But only one of their phones lets me send cash through Apple Cash. I set both accounts up the exact same way - able to send money and receive to “family members only”. When I attempt to send money to the one child’s phone, I get a message saying they “cannot receive payments with Apple Cash at this time.” But the other child is able to receive. Can someone help me?