what u said is correct. but we cant depend our ISP always. whey they try to block sites in one way, people will find more ways to get it indirectly. Since IT is very dynamic area, new new things and technologies are coming.
And there are lots of people who are spending their time, may be as a hobby or some thing other, to bypass the filter or similar activities.
so, its parents responsibility to keep their children away from GOING TOWARDS THE BAD SIDE OF INTERNET.