Children Unreturned to Australia After Israeli Holiday: My children went to visit their father (my 'ex') in Israel for the recent Jewish festival. He now refuses to return them to Australia, where I have custody of them, according to our divorce agreement. What can I do?

Bring a legal action under the Hague Convention for their return. From what you say, their father has ‘wrongfully retained’ them in Israel, while Australia is their country of habitual residence under the Convention. You have a good chance of succeeding, on the face of it.
Abduction To Israel