Screen time passcode not updating on my child's iPhone

My child found out what the parental screen time passcode was on her phone so we needed to change it. I have now changed it on my device, following the instructions given in previous threads about this issue, but it has not updated on my child's device and she continues t be able to override any screen time restriction using the old passcode.

So far I have tried the following:

Turned both phones off and on again after updating the passcode - didn't work, and I tried that several times!

I have tried removing the passcode altogether and then turning it back on again - the first time I did this it resulted in her not needing a passcode at all for a time, then when the requirement for a passcode finally did start up on her device it was still the old passcode that she knows, and so was useless. I've just done that for a second time (after iOS updates) and she currently doesn't need a passcode at all...

Both devices (mine and my child's) have been updated to the latest iOS (18.1)

Can you please help because at the moment the screen time restrictions I've put in place for my 12 year old are absolutely pointless!