Hi Sue & Ian
I have just had a look at the checklist for the subclass 572 visa. I agree that it does seem to say that they want evidence that your children are enrolled in an Australian school etc.
The main stuff about the visa indicates that you should ask the Australian High Commission in London about this. Please see here:
United Kingdom - Contacts
I think I would be inclined to phone, fax or e-mail them asking them how this notion is supposed to work in the circumstances which you describe.
Best wishes
Gill