It is a silly law once again empowering external childcare over parents.
It is very debatable whether it makes much difference to how much smacking goes on (has been illegal in a Sweden since 70s and plenty still smack there).
It also encourages an informer culture (see some posts above). Of course if you see a child beaten it should be reported but a single smack?! Equally children will be guilted in not discussing smacking with outsiders (‘if you tell mummy will go to jail’). Again apparently this happens in Sweden.
One of those laws which makes lefties feel good but has plenty of unintended consequences, mostly negative to children who really need help.