The problem is that Parental Controls rely on filtering websites using DNS (Domain Name System), and not the content itself. Your data doesn't pass through the DNS, rather DNS tells your browsers, apps, and other stuff where things are. In the case of filtering what you don't want access to, DNS points to a dead end rather than where that data actually is.
The problem with this approach is that it relies on the filtered DNS actually being used, which the BT routers do not (for good reason) enforce. What can go wrong is that devices and/or don't use the system-advertised DNS, leading to the filtering being bypassed.
It's possible to rattle on about this for pages, but it's probably better to let the thread develop naturally.
In the meantime have a look at what DNS Leak Test reports your DNS servers as being - they should be BT. Please don't get too paranoid about DNS Leak Tests, in order for your devices to work they need to be sending requests to some DNS servers.
Always use caution when following external links!
I only learn by making mistakes and owning up to them - boy do I learn a lot!
