There is no obligation to pixellate anything unless there is a court order, or the pictures are taken on private property or in a country such as France which has an actual privacy law.
In reality France is like the Wild West if the money is good enough.
Some people are hypocrites. Those pix of Kate Middleton topless were a case in point. They were taken in a private place via a long lens from a very long way on a public road.
I thought she was entitled to expect privacy under all the circumstances but it wasn't worth kicking a fuss up about it, and yet the overwhelming MN view was that she should have kept her top on 
But if the picture is taken outside those areas then they might choose to pixellate for other reasons.
With children it's chiefly that the parent is litigious and might be able to argue a real or imagined case for child protection. That case would be strong if you were Jude Law and Sadie Frost and somewhat diminished if you are David and Victoria Beckham or Kanye West and Kim Kardashian.
Much as people dislike her, Gwyneth Paltrow's children are generally pixellated because she doesn't use them for publicity. Same with the hated Keira Knightley.
I don't think newspapers should routinely pixellate any images of people of whatever age taken on public property.
I do think they should take reasonable requests to desist into account and avoid publishing pictures of distress provoked by photographers - though not in all circumstances.