I have spent a long time on the line to Apple support about this, but as I could not re-create the error at the time - not surprising when the problem is intermittent - I could not progress the case forward.
Apple support requested I do something to address: Immediately after noticing the problem, make contact with them, so they can analyse the logs on the device I am on from the previous minutes.
There are a few problems with this - getting escalated to someone who trusts that you aren't making a bone-headed mistake is not easy at all. Tier 1 support staff ask the most tedious questions, seemingly based on the premise that most problems are caused by user-error... So, you need time and patience to make the report at the time it happens, when all that's really on my mind is securing the iPhones, iPads, MacBooks and HomePods my kids have access to.
Further, I'm skeptical that the logs of the precise device I am using at the time will show anything - who knows which device made the change? And when? I'm not an expert, so perhaps the device's own logs would be useful, but there are a multitude of devices involved in the problem and there's no way of knowing for how long the settings have been wrong.
This is a serious problem for families who trust that the Apple products and services they are using and paying for have their back when it comes to kids' online safety.
Please, when you notice this, call Apple about it. Who knows how well-monitored this forum is by staff, and we need multiple voices letting Apple know there is a serious problem.