That's the problem though. Unless you ban people from the internet entirely and shut down all the ISPs then you're not going to stop it. I can guarantee you that the first time you put a block out there at ISP level that your kids will be told by their friends at school about proxies, VPNs and darknet/ToR, with many of them utterly untrackable except in the most extreme circumstances, and even then it's not guaranteed.
Those are the things that are used now by people from those just paranoid about their privacy through to the most extreme edge of criminality out there. They know it's safe and almost unbreakable.
For those things the ISP will ban, how will they do it? Based on website addresses? It takes seconds to set up a new one. Who says a website goes on the list? What if a politician decides that the BNP website is unsafe for kids? Block that too? What if then the next step is to completely ban these things for everyone?
Look at China, it's paranoid to the extreme about what its citizens can see on the internet but they've all bar given up trying to censor things because the availability of the bypass technologies are so ubiquitous that anyone who needs to use them can get them. It's not rocket science to get around virtually any ISP level blocking tools.
That's why I'm quite happy with things as they stand. It puts responsibility on the parents to monitor their kids' internet usage. What kid would go to his friends and tell them that his mummy and daddy had put child protection software on their computers? If you can afford to buy a computer then you can afford to buy this software and learn how to use it.
My friend allows his daughter unrestricted access to whatever she wants on the internet but has the computer configured so she can't delete browsing history, he tells her he reads this once a week and checks her emails at the same time. He has her messenger set up to verbosely duplicate messages received or sent on her iPhone on his computer. She grumbled about how unfair it was but was bluntly told that it was her choice if she didn't want to use them. He told me he actually randomly scans her browser history once a month or so when he remembers but the threat of it alone is enough to bluntly force good behaviour. Also, as he has the computer locked down he can see any bypass tools she'd install or such websites visited, he also has the privacy mode disabled on the computer's browsers.
Most Chinese people don't use facebook. A lot of them have the skills to get around the "great firewall of China" but there is no particular point in doing so when there is an ap that is equivalent to facebook and isn't blocked.