Australian migration - help!!!!?

I am an Australian citizen living in Sydney. My de facto partner is in Europe and wants to move here to be with me, so he's applying for a de facto partner visa. It says on the form that he has to have a medical, which is no problem, but it also says that his children (he has two from a previous marriage - now teenagers) also have to have medicals. They wont be migrating with him - they live with their mother and refuse to even see him. His ex-wife is extremely difficult to deal with and will, without doubt, refuse to give her permission for the children to attend a medical and, even if she does, then the children will almost certainly refuse - having been brainwashed by her against their father!!! Is there any way around this - could he swear an affidavit for instance? Is it an absolute hard and fast rule that they'll have to undergo the medical? Would his whole application be put in jeopardy if they don't have the medical???