I am outraged to find out I am trapped in the same problem. I spent couple of my precious days to figure out how to play with family settings with Microsoft account and got my 13 years old son to be added into my family.
The screen time limit etc just worked for 2 weeks before he found out this notorious "opt out" button in his account last night. I did get an email notification showing your son left your family but how come! It is a such bad design and useless feature that Microsoft has been bragging about that your kid can just simply get the escape with 1 click without the consent from the family organizer, parents. The most absurd thing is that It didn't cost me more than 1 minute to google out these many threads complaining the loophole discussed by so many pissed off parents for quite a number of years since 2015! What a shame to Microsoft 's PM, Dev, and QA on this. Think about I even need to ask my child to agree to accept my invite of joining him into the family, then what 's the heck that I was not even being asked to agree when he want to leave?
This ridiculous 13 years line reminds me of another anti-human policy that you don't have access to your children's online medical account even for the purpose of making appointment or paying statements within for the stupid reasons. If this is the case, why not directly bill them instead of always sending bills to parents!? I need to think of moving to PS or Nintendo systems if they have better parental control.