Actually what makes it difficult in the UK are rulings from the European Court of Human Rights – not rulings of the ICJ. The ECHR is not part of the EU, but predates it.
Both Australia and the US are signatories to the Universal Declaration of Human Rights, but this does not ban corporal punishment of children.
The US is not a signatory to the Declaration of the Rights of the Child – Australia is. But this Declaration does not ban the corporal punishment of children even if a bunch of UN bureaucrats persistently try to claim it does by creatively interpreting certain passages.