Family cloud/sharing with single iphone in a family

Wow. My son brough a iphone. None of us in family use apple products. So controlin his streen time is a challnage. I though I will be able to use my old IPAD 4 with old iOS at the time a family cloud/sharing wasn't a thing

We anyway somehow manage to set it up. He got to know my family PIN so I though I will change it... but it seems to be impossible. No such options, and managign the account/family from website doesn't exist.

So seems like I have no option to use my business iphon to add family sharing/cloud, but how again I move my child account between parent/family if only option is thorugh family sharign

I stuck. How to manage a chile screen time and security if this can't be manage from web/other device.

Any advices highly welcome.