"1) It probably will work"
No it won't. Even China who are desperate to censor the internet and throw vast amounts of human and financial resources at the "problem" can't make it work. Plus China don't mind "accidentally" blocking entire sites that cause a problem.
"2) It'll cost ISPs money but I'm sure they can cope with that"
There's not a lot of money in the ISP game, it's an expensive business. They will just pass all the costs over to the users, so internet costs will rise pricing people off of the internet.
"3)Children are already unsafe and being exposed to pornography. Parents unfortunately aren't protecting them or rather aren't able to protect them because the stuff is everywhere now."
True.
So the solution is to educate people out of their position of ignorance so they can protect their children.
Putting in place a system that, as said, won't work will just trick those that are trying to keep their guard into thinking "well I don't need to do anything now" when they still need to be on their toes.
Any delay to the point in time when we begin to educate people about the internet is time in which children will be exposed to harmful content on the internet.
"4)Don't tell me what I should or shouldn't be doing, that's just cheeky."
Well you seem to want to make a difference and to do good things, and this proposal is the complete opposite of that.