And the definition of 'porn' is?
This would imply an official classification of exactly what is and is not 'porn'. That's possible for limited cases like films, but is ludicrous for the Internet.
Whatever line they draw, someone is going to think it doesn't go far enough and will scream about the government letting their child see obscenities (because once this is in place, every failure of it is the fault of the authorities and Something Must Be Done!). "You let my child see pictures of unveiled women!".
So you get mission creep. You include more and more things because someone somewhere found them objectionable and complained that their children were being harmed.
How far do you go? Do you include everything that anyone ever found sexually stimulating? "Some people are into sheep, and you let my children see pictures of a farm!"
At which point you run headlong into Rule 34.