how to stop settings being changed on a child’s iPhone. Can’t do it under privacy settings or family sharing settings

Hello and welcome to Apple Support Communities, Robbo2008!

We understand you want to prevent Settings from being changed on your child's iPhone. We'd be happy to provide information to help you address this.

Make sure you've changed the settings to not allow changes without the passcode:

Use parental controls on your child's iPhone, iPad, and iPod touch

Have a great day!

Hey Robbo2008!

On your child's iPhone, if you go to Settings > Screen Time > Content & Privacy Restrictions and change the options there to "Don't Allow Changes" does this block what you are trying to block?

Regards.