Cannot set up Apple Cash for child, with family sharing enabled

My wife is the family manager. She enabled apple cash for our youngest. she got a confirmation on her phone that the account was set up. i sent $40 to him via apple cash in text.

when my son opens his phone, it asks him to set up Apple Cash. Clicking continue gives the error "Apple Pay Unavailable"

So the $40 is now just in limbo and my son still cannot use his phone to pay for anything.

We have restarted the phone, restarted the wifi and checked his app settings. all look good.