Hope they get better!
I don't know if you can get it in Oz, but 5% [50mg/ml] toltrazuril (generic for Baycox) works much faster than Corid.
Give it by mouth 7-10 mg/kg body weight. The protocol I use is to dose for 2 days, repeat in a week, or when symptoms return.
If you are dosing lots of birds, you can give it in the drinking water.
This vendor appears to ship internationally https://racehorsemeds.com/product-category/toltrazuril/
You want the 5% oral suspension.
If your babies are skin & bones and pooping blood, it may be too late.