This happened while they were swimming at a beach at Pt Samson in the Pilbara, WA, see;

http://www.abc.net.au/news/2015-01-06/two-children-stung-by-deadly-irukandji-jellyfish/6002034 

Anyone living up north should by now know "don't go to beach with out a big/huge bottle of vinegar".

As the doctor said:"If there's suspicion of a sting, vinegar should be applied as first aid if available, to the sting site,"

Lived in Darwin for several years. The bottle of vinegar was part of the "go to beach" preps.

Blame the parents for their basic stupidity here...