Woolworths sells HAZARDOUS yoghurt (And wont refund for it)

I bought a six pack of full cream yoghurts on 4/8, the expiry of the yoghurt was 12/8. All of yoghurts had gone off. One of these was sent with my 3 year old son to school, which he ate and felt ill as a result. I have just been told be the store i bought them that they wont refund me because I need to bring the yoghurts in. I have already thrown away most of them because they were opened and found the be off. This is absolutely disgraceful behaviour, not to mention a health hazard.