I don't think childrens rights undermines parental discipline at all. Physically hitting children isn't discipline - its badly vented anger and frustration - if a bigger kid were to hit your child you'd say how unfair it was - what kind of reasoning is it then for fully grown adults to hit children?!
No doubt it is hard to discipline children, but I think its right to say that parents should back up schools and other organisations that attempt to discipline children and young people.