How to stop child changing screen time limits

My son was using an ipad that was set up using his Dad's apple id. I've just reset it so it's set up with my son's apple id, thinking it would be better for restricting adult content and screen time etc. However there is no option to put a passcode on the screen time which means whatever I set up, he can go in and change. When the ipad was set up with his dad's Apple ID, you had to put in a passcode to access the screen time settings. Now it's no longer there.

I can also no longer download Google Chrome (which restricted adult websites because he's under 13) because the iPad doesn't have ios 17. Safari still allows access to adult sites giving the option to simply 'allow website' if he goes to one that's restricted.

I now have an ipad that basically has zero way of restricting use or content accessibility for an 11 year old.

Why is there no way of locking screen time settings anymore, and how can I get a browser for iOS 16 that actually blocks adult sites without the option of overriding?