Firstly as a parent bringing up children in the newly connected world, I can state from my own experience that a default filter will not work. The first thing will be that parents will feel they have absolved of the need to educate their children on Internet safety. Second that the filter will not work, in that it will block perfectly reasonable sites and fail to block highly objectionable content.
As a Security Expert in the internet age I have more exposure that most to these issues, while some things are best solved with technology, there are many security issues that are far better solved by humans taking a critical look at the content and evaluating it.
Take for example the recent Facebook meam on Bananas and tumor necrosis factor. On the face of it many have been convinced that there is a benefit here, but in fact the truth is that this linkage in the science is extremely tenuous, and based on implanting banana rather than eating it.
I use this innocuous example to show that no technological firewall would be able to block this message, and that we are better off teaching skills in critical evaluation of information than trying to get into the arms race of attempting to block content.
The Music and Video industries have been trying for years to block music and video sharing on the internet using all of the resources of that industry and by any reasonable measure they have failed miserably. Are you really prepared to invest many times more effort in blocking content on the internet.
What has worked well is the operation of the Internet Watch Foundation, in terms of providing a central reporting point, and coordinating take down activities of content that we, as a society, find highly objectionable. But if you read their reports you will see that their attempts to block sites has not been as successful.
This point is at the route of the problem of the proposals, if the material is highly objectionable then it needs to be taken down at source, not blocked. Blocks can always be bypassed. If we are taking down material that is a legal process, with appropriate appeals processes. What you are proposing would be arbitrary and secret, without a transparency on the blocked sites so how would a site owner know they were being blocked, how would they appeal against that, how would we know that the process of blocking sites was fair and reasonable and not being used for political ends.