My understanding is once Apple Cash Family is setup a minor child can send and receive funds via Apple Cash. Restrictions can be enabled to restrict who they send money to.
Unfortunately, I don’t have a child or grandchild young enough to test this. Please review the support article I’ve linked to below and confirm what step the send is failing at? Have you confirmed eligibility of the devices involved?
>> Set up and use Apple Cash Family
Learn what you need to set up and use Apple Cash for members of your Family Sharing group that are under 18 years old.
What is Apple Cash Family?
You can set up Apple Cash for the children and teens in your Family Sharing group so they can make purchases, and send and receive money in Messages or Wallet. You can even limit who your child can send money to, get notified when they make transactions, and lock their account. <<
Set up and use Apple Cash Family - Apple Support